ext4: force inode writes when nfsd calls commit_metadata()

Some time back, nfsd switched from calling vfs_fsync() to using a new
commit_metadata() hook in export_operations().  If the file system did
not provide a commit_metadata() hook, it fell back to using
sync_inode_metadata().  Unfortunately doesn't work on all file
systems.  In particular, it doesn't work on ext4 due to how the inode
gets journalled --- the VFS writeback code will not always call
ext4_write_inode().

So we need to provide our own ext4_nfs_commit_metdata() method which
calls ext4_write_inode() directly.

Google-Bug-Id: 121195940
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/super.c

@@ -1202,6 +1202,16 @@ static struct dentry *ext4_fh_to_parent(struct super_block *sb, struct fid *fid,
 				    ext4_nfs_get_inode);
 }
 
+static int ext4_nfs_commit_metadata(struct inode *inode)
+{
+	struct writeback_control wbc = {
+		.sync_mode = WB_SYNC_ALL
+	};
+
+	trace_ext4_nfs_commit_metadata(inode);
+	return ext4_write_inode(inode, &wbc);
+}
+
 /*
  * Try to release metadata pages (indirect blocks, directories) which are
  * mapped via the block device.  Since these pages could have journal heads
@@ -1406,6 +1416,7 @@ static const struct export_operations ext4_export_ops = {
 	.fh_to_dentry = ext4_fh_to_dentry,
 	.fh_to_parent = ext4_fh_to_parent,
 	.get_parent = ext4_get_parent,
+	.commit_metadata = ext4_nfs_commit_metadata,
 };
 
 enum {