bpf: Send signals asynchronously if !preemptible

[ Upstream commit 87c544108b612512b254c8f79aa5c0a8546e2cc4 ]

BPF programs can execute in all kinds of contexts and when a program
running in a non-preemptible context uses the bpf_send_signal() kfunc,
it will cause issues because this kfunc can sleep.
Change `irqs_disabled()` to `!preemptible()`.

Reported-by:  <syzbot+97da3d7e0112d59971de@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/67486b09.050a0220.253251.0084.GAE@google.com/


Fixes: 1bc7896e ("bpf: Fix deadlock with rq_lock in bpf_send_signal()")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250115103647.38487-1-puranjay@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>