fanotify: Fix crash in fanotify_init(2)

The rrror handling in fanotify_init(2) is buggy and overwrites 'fd'
before calling put_unused_fd() leading to possible access beyond the end
of fd bitmap. Fix it.

Reported-by: syzbot+6a3aa63412255587b21b@syzkaller.appspotmail.com
Fixes: ebe5596 ("fs: get rid of __FMODE_NONOTIFY kludge")
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fanotify/fanotify_user.c

@@ -1624,8 +1624,8 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 	file = anon_inode_getfile_fmode("[fanotify]", &fanotify_fops, group,
 					f_flags, FMODE_NONOTIFY);
 	if (IS_ERR(file)) {
-		fd = PTR_ERR(file);
 		put_unused_fd(fd);
+		fd = PTR_ERR(file);
 		goto out_destroy_group;
 	}
 	fd_install(fd, file);