switch a bunch of places to mnt_want_write_file()

it's both faster (in case when file has been opened for write) and cleaner.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/ubifs/ioctl.c b/fs/ubifs/ioctl.c
index 548acf4..e52c845 100644
--- a/fs/ubifs/ioctl.c
+++ b/fs/ubifs/ioctl.c
@@ -173,7 +173,7 @@
 		 * Make sure the file-system is read-write and make sure it
 		 * will not become read-only while we are changing the flags.
 		 */
-		err = mnt_want_write(file->f_path.mnt);
+		err = mnt_want_write_file(file);
 		if (err)
 			return err;
 		dbg_gen("set flags: %#x, i_flags %#x", flags, inode->i_flags);