UBIFS: use ro_mount instead of MS_RDONLY

We have our own flags indicating R/O mode, and c->ro_mode is equivalent
to MS_RDONLY. Let's be consistent and use UBIFS flags everywhere.
This patch is just a minor cleanup.

Additionally, add a comment that we are surprised with VFS behavior -
as a reminder to look at this some day.
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/file.c

@@ -1312,7 +1312,11 @@ int ubifs_fsync(struct file *file, int datasync)
 
 	dbg_gen("syncing inode %lu", inode->i_ino);
 
-	if (inode->i_sb->s_flags & MS_RDONLY)
+	if (c->ro_mount)
+		/*
+		 * For some really strange reasons VFS does not filter out
+		 * 'fsync()' for R/O mounted file-systems as per 2.6.39.
+		 */
 		return 0;
 
 	/*