bpf: Fix mmap_lock leak in irq_work path

stack_map_get_build_id_offset() introduced a per-CPU irq_work to defer
mmap_read_unlock() from NMI context, and bpf_find_vma() later reused the
same mmap_unlock_work. Both callers only check whether the work is busy
before taking mmap_lock, so a nested caller can reuse the slot before the
first caller queues it. Two read locks may then be acquired while only one
deferred unlock runs, leaking a read lock and blocking exit_mmap().

Reserve the per-CPU slot before mmap_read_trylock(). Use the same wrapper
in stackmap and bpf_find_vma() so both callers release the reservation on
trylock failure. Keep rejecting the slot while the irq_work remains busy.
Release it after the irq_work callback unlocks the mm.

Fixes: eac9153f2b58 ("bpf/stackmap: Fix deadlock with rq_lock in bpf_get_stack()")
Reported-by: syzbot+cdd6c0925e12b0af60cc@syzkaller.appspotmail.com
Reported-by: sashiko-bot@kernel.org
Signed-off-by: Sanghyun Park <sanghyun.park.cnu@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Closes: https://syzkaller.appspot.com/bug?extid=cdd6c0925e12b0af60cc
Closes: https://lore.kernel.org/r/20260630033745.B80201F000E9@smtp.kernel.org
Link: https://lore.kernel.org/bpf/20260805031425.2157475-2-sanghyun.park.cnu@gmail.com

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