9p: Include fsync support for 9p client

Implement the fsync in the client side by marking stat field values to 'don't touch' so that server may 
interpret it as a request to guarantee that the contents of the associated file are committed to stable 
storage before the Rwstat message is returned.

Without this patch, calling fsync on a 9p file results in "Invalid argument" error. Please check the attached 
C program.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> 
Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com> 
Acked-by: NVenkateswararao Jujjuri (JV) <jvrao@linux.vnet.ibm.com>
Signed-off-by: NEric Van Hensbergen <ericvh@gmail.com>

fs/9p/v9fs_vfs.h

@@ -60,3 +60,4 @@ void v9fs_dentry_release(struct dentry *);
 int v9fs_uflags2omode(int uflags, int extended);
 
 ssize_t v9fs_file_readn(struct file *, char *, char __user *, u32, u64);
+void v9fs_blank_wstat(struct p9_wstat *wstat);

fs/9p/vfs_file.c

@@ -257,6 +257,23 @@ v9fs_file_write(struct file *filp, const char __user * data,
 	return total;
 }
 
+static int v9fs_file_fsync(struct file *filp, struct dentry *dentry,
+					int datasync)
+{
+	struct p9_fid *fid;
+	struct p9_wstat wstat;
+	int retval;
+
+	P9_DPRINTK(P9_DEBUG_VFS, "filp %p dentry %p datasync %x\n", filp,
+						dentry, datasync);
+
+	fid = filp->private_data;
+	v9fs_blank_wstat(&wstat);
+
+	retval = p9_client_wstat(fid, &wstat);
+	return retval;
+}
+
 static const struct file_operations v9fs_cached_file_operations = {
 	.llseek = generic_file_llseek,
 	.read = do_sync_read,
@@ -266,6 +283,7 @@ static const struct file_operations v9fs_cached_file_operations = {
 	.release = v9fs_dir_release,
 	.lock = v9fs_file_lock,
 	.mmap = generic_file_readonly_mmap,
+	.fsync = v9fs_file_fsync,
 };
 
 const struct file_operations v9fs_file_operations = {
@@ -276,4 +294,5 @@ const struct file_operations v9fs_file_operations = {
 	.release = v9fs_dir_release,
 	.lock = v9fs_file_lock,
 	.mmap = generic_file_readonly_mmap,
+	.fsync = v9fs_file_fsync,
 };

fs/9p/vfs_inode.c

@@ -176,7 +176,7 @@ int v9fs_uflags2omode(int uflags, int extended)
  *
  */
 
-static void
+void
 v9fs_blank_wstat(struct p9_wstat *wstat)
 {
 	wstat->type = ~0;