f2fs: fix error handling in fill_super

In fill_super, if we fail to call f2fs_build_stats(), it needs to detach
from global f2fs shrink list, otherwise once system starts to shrink slab
cache, we will encounter below panic:

BUG: unable to handle kernel paging request at 00007d35
Oops: 0002 [#1] PREEMPT SMP
EIP: __lock_acquire+0x70/0x12c0
Call Trace:
 lock_acquire+0xae/0x220
 mutex_trylock+0xc5/0xf0
 f2fs_shrink_count+0x32/0xb0 [f2fs]
 shrink_slab+0xf1/0x5b0
 drop_slab_node+0x35/0x60
 drop_slab+0xf/0x20
 drop_caches_sysctl_handler+0x79/0xc0
 proc_sys_call_handler+0xa4/0xc0
 proc_sys_write+0x1f/0x30
 __vfs_write+0x24/0x150
 SyS_write+0x44/0x90
 do_fast_syscall_32+0xa1/0x1ca
 entry_SYSENTER_32+0x4c/0x7b

In addition, this patch relocates f2fs_join_shrinker in fill_super to
avoid unneeded error handling of it.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -2615,18 +2615,16 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 		goto free_nm;
 	}
 
-	f2fs_join_shrinker(sbi);
-
 	err = f2fs_build_stats(sbi);
 	if (err)
-		goto free_nm;
+		goto free_node_inode;
 
 	/* read root inode and dentry */
 	root = f2fs_iget(sb, F2FS_ROOT_INO(sbi));
 	if (IS_ERR(root)) {
 		f2fs_msg(sb, KERN_ERR, "Failed to read root inode");
 		err = PTR_ERR(root);
-		goto free_node_inode;
+		goto free_stats;
 	}
 	if (!S_ISDIR(root->i_mode) || !root->i_blocks || !root->i_size) {
 		iput(root);
@@ -2722,6 +2720,8 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 			sbi->valid_super_block ? 1 : 2, err);
 	}
 
+	f2fs_join_shrinker(sbi);
+
 	f2fs_msg(sbi->sb, KERN_NOTICE, "Mounted with checkpoint version = %llx",
 				cur_cp_version(F2FS_CKPT(sbi)));
 	f2fs_update_time(sbi, CP_TIME);
@@ -2748,14 +2748,12 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 free_root_inode:
 	dput(sb->s_root);
 	sb->s_root = NULL;
+free_stats:
+	f2fs_destroy_stats(sbi);
 free_node_inode:
-	truncate_inode_pages_final(NODE_MAPPING(sbi));
-	mutex_lock(&sbi->umount_mutex);
 	release_ino_entry(sbi, true);
-	f2fs_leave_shrinker(sbi);
+	truncate_inode_pages_final(NODE_MAPPING(sbi));
 	iput(sbi->node_inode);
-	mutex_unlock(&sbi->umount_mutex);
-	f2fs_destroy_stats(sbi);
 free_nm:
 	destroy_node_manager(sbi);
 free_sm: