vfs: mnt_drop_write_file()

new helper (wrapper around mnt_drop_write()) to be used in pair with
mnt_want_write_file().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ubifs/ioctl.c

@@ -178,7 +178,7 @@ long ubifs_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			return err;
 		dbg_gen("set flags: %#x, i_flags %#x", flags, inode->i_flags);
 		err = setflags(inode, flags);
-		mnt_drop_write(file->f_path.mnt);
+		mnt_drop_write_file(file);
 		return err;
 	}