bpf: Fix mmap_lock deadlock on arena lock failure

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix mmap_lock deadlock on arena lock failure

Reported by the Sashiko AI review.

arena_vm_fault() returns VM_FAULT_RETRY when it can't take
arena->spinlock, but it never took mmap_lock. The fault path assumes a
VM_FAULT_RETRY handler already dropped mmap_lock and re-takes it on the
retry, so mmap_lock gets taken twice and can deadlock:

do_user_addr_fault()
{
fault = handle_mm_fault(...); // calls arena_vm_fault()
if (fault & VM_FAULT_RETRY)
goto retry; // re-locks mmap_lock
mmap_read_unlock(mm);
}

Return VM_FAULT_SIGBUS instead, for two reasons:

1. We could keep VM_FAULT_RETRY, but then we'd have to drop the fault
lock first and cap the retry ourselves, the way __folio_lock_or_retry()
does.

2. A failed raw_res_spin_lock_irqsave() already means a possible deadlock
was detected, so retrying just hits the same lock again.

So returning VM_FAULT_RETRY here is overkill.