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/ncpfs/ioctl.c b/fs/ncpfs/ioctl.c
index 790e92a..6958adf 100644
--- a/fs/ncpfs/ioctl.c
+++ b/fs/ncpfs/ioctl.c
@@ -901,7 +901,7 @@
 	ret = __ncp_ioctl(inode, cmd, arg);
 outDropWrite:
 	if (need_drop_write)
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 out:
 	return ret;
 }