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/gfs2/file.c b/fs/gfs2/file.c
index 28fc6e3..b8927d4 100644
--- a/fs/gfs2/file.c
+++ b/fs/gfs2/file.c
@@ -285,7 +285,7 @@
 out:
 	gfs2_glock_dq_uninit(&gh);
 out_drop_write:
-	mnt_drop_write(filp->f_path.mnt);
+	mnt_drop_write_file(filp);
 	return error;
 }