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>
diff --git a/fs/ubifs/ioctl.c b/fs/ubifs/ioctl.c
index e52c845..1a7e2d8 100644
--- a/fs/ubifs/ioctl.c
+++ b/fs/ubifs/ioctl.c
@@ -178,7 +178,7 @@
 			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;
 	}