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/open.c b/fs/open.c
index 22c41b5..4ef8d86 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -608,7 +608,7 @@
 	dentry = file->f_path.dentry;
 	audit_inode(NULL, dentry);
 	error = chown_common(&file->f_path, user, group);
-	mnt_drop_write(file->f_path.mnt);
+	mnt_drop_write_file(file);
 out_fput:
 	fput(file);
 out: