bpf: Fix bad unlock balance on freeze_mutex

Commit c4c84f6fb2 ("bpf: drop unnecessary bpf_capable() check in
BPF_MAP_FREEZE command") moved the permissions check outside of the
freeze_mutex in the map_freeze() handler. The error paths still jumps
to the err_put which tries to unlock the freeze_mutex even though it
was not locked in the first place. Fix it.

Fixes: c4c84f6fb2 ("bpf: drop unnecessary bpf_capable() check in BPF_MAP_FREEZE command")
Reported-by: syzbot+8982e75c2878b9ffeac5@syzkaller.appspotmail.com
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/syscall.c

@@ -1932,8 +1932,8 @@ static int map_freeze(const union bpf_attr *attr)
 	}
 
 	if (!(map_get_sys_perms(map, f) & FMODE_CAN_WRITE)) {
-		err = -EPERM;
-		goto err_put;
+		fdput(f);
+		return -EPERM;
 	}
 
 	mutex_lock(&map->freeze_mutex);