unix_bpf: Fix a potential deadlock in unix_dgram_bpf_recvmsg()

As Eric noticed, __unix_dgram_recvmsg() may acquire u->iolock
too, so we have to release it before calling this function.

Fixes: 9825d866

 ("af_unix: Implement unix_dgram_bpf_recvmsg()")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>