CacheFiles: Mark parent directory locks as I_MUTEX_PARENT to keep lockdep happy

Mark parent directory locks as I_MUTEX_PARENT in the callers of
cachefiles_bury_object() so that lockdep doesn't complain when that invokes
vfs_unlink():

=============================================
[ INFO: possible recursive locking detected ]
2.6.32-rc6-cachefs #47
---------------------------------------------
kslowd002/3089 is trying to acquire lock:
 (&sb->s_type->i_mutex_key#7){+.+.+.}, at: [<ffffffff810bbf72>] vfs_unlink+0x8b/0x128

but task is already holding lock:
 (&sb->s_type->i_mutex_key#7){+.+.+.}, at: [<ffffffffa00e4e61>] cachefiles_walk_to_object+0x1b0/0x831 [cachefiles]

other info that might help us debug this:
1 lock held by kslowd002/3089:
 #0:  (&sb->s_type->i_mutex_key#7){+.+.+.}, at: [<ffffffffa00e4e61>] cachefiles_walk_to_object+0x1b0/0x831 [cachefiles]

stack backtrace:
Pid: 3089, comm: kslowd002 Not tainted 2.6.32-rc6-cachefs #47
Call Trace:
 [<ffffffff8105ad7b>] __lock_acquire+0x1649/0x16e3
 [<ffffffff8118170e>] ? inode_has_perm+0x5f/0x61
 [<ffffffff8105ae6c>] lock_acquire+0x57/0x6d
 [<ffffffff810bbf72>] ? vfs_unlink+0x8b/0x128
 [<ffffffff81353ac3>] mutex_lock_nested+0x54/0x292
 [<ffffffff810bbf72>] ? vfs_unlink+0x8b/0x128
 [<ffffffff8118179e>] ? selinux_inode_permission+0x8e/0x90
 [<ffffffff8117e271>] ? security_inode_permission+0x1c/0x1e
 [<ffffffff810bb4fb>] ? inode_permission+0x99/0xa5
 [<ffffffff810bbf72>] vfs_unlink+0x8b/0x128
 [<ffffffff810adb19>] ? kfree+0xed/0xf9
 [<ffffffffa00e3f00>] cachefiles_bury_object+0xb6/0x420 [cachefiles]
 [<ffffffff81058e21>] ? trace_hardirqs_on+0xd/0xf
 [<ffffffffa00e7e24>] ? cachefiles_check_object_xattr+0x233/0x293 [cachefiles]
 [<ffffffffa00e51b0>] cachefiles_walk_to_object+0x4ff/0x831 [cachefiles]
 [<ffffffff81032238>] ? finish_task_switch+0x0/0xb2
 [<ffffffffa00e3429>] cachefiles_lookup_object+0xac/0x12a [cachefiles]
 [<ffffffffa00741e9>] fscache_lookup_object+0x1c7/0x214 [fscache]
 [<ffffffffa0074fc5>] fscache_object_state_machine+0xa5/0x52d [fscache]
 [<ffffffffa00754ac>] fscache_object_slow_work_execute+0x5f/0xa0 [fscache]
 [<ffffffff81082093>] slow_work_execute+0x18f/0x2d1
 [<ffffffff8108239a>] slow_work_thread+0x1c5/0x308
 [<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34
 [<ffffffff810821d5>] ? slow_work_thread+0x0/0x308
 [<ffffffff8104be91>] kthread+0x7a/0x82
 [<ffffffff8100beda>] child_rip+0xa/0x20
 [<ffffffff8100b87c>] ? restore_args+0x0/0x30
 [<ffffffff8104be17>] ? kthread+0x0/0x82
 [<ffffffff8100bed0>] ? child_rip+0x0/0x20
Signed-off-by: NDaivd Howells <dhowells@redhat.com>

fs/cachefiles/namei.c

@@ -254,7 +254,7 @@ int cachefiles_delete_object(struct cachefiles_cache *cache,
 
 	dir = dget_parent(object->dentry);
 
-	mutex_lock(&dir->d_inode->i_mutex);
+	mutex_lock_nested(&dir->d_inode->i_mutex, I_MUTEX_PARENT);
 	ret = cachefiles_bury_object(cache, dir, object->dentry);
 
 	dput(dir);
@@ -307,7 +307,7 @@ int cachefiles_walk_to_object(struct cachefiles_object *parent,
 	/* search the current directory for the element name */
 	_debug("lookup '%s'", name);
 
-	mutex_lock(&dir->d_inode->i_mutex);
+	mutex_lock_nested(&dir->d_inode->i_mutex, I_MUTEX_PARENT);
 
 	start = jiffies;
 	next = lookup_one_len(name, dir, nlen);