bpf: Fix bad unlock balance on freeze_mutex

Commit c4c84f6f ("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: c4c84f6f

 ("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>