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/fat/file.c b/fs/fat/file.c
index 50746a1..d81d01a 100644
--- a/fs/fat/file.c
+++ b/fs/fat/file.c
@@ -108,7 +108,7 @@
 	fat_save_attrs(inode, attr);
 	mark_inode_dirty(inode);
 out_drop_write:
-	mnt_drop_write(file->f_path.mnt);
+	mnt_drop_write_file(file);
 out_unlock_inode:
 	mutex_unlock(&inode->i_mutex);
 out: