selftests/bpf: Test bpf_send_signal_task() from kernel thread context

The existing send_signal tests, including the _remote variants that
exercise bpf_send_signal_task() with a caller-supplied target, all
gate on

  if ((bpf_get_current_pid_tgid() >> 32) == pid)

so the BPF program only ever runs while current is the test process.
The caller-context dimension is therefore untested, which is how
bpf_send_signal_task() failing from kernel thread context went
unnoticed.

Add a program attached to tp_btf/workqueue_execute_start, where
current is always a kworker. It cannot filter on current, so it is
driven by target_pid and gated on a kworker_test flag that only this
test sets, and the test asserts the child receives the signal.
Setting pid to 0 leaves the pre-existing programs in the skeleton
inert. Workqueues run on their own, so nothing has to be triggered
from userspace.

Without removing the nmi_uaccess_okay() check, this test fails on x86.

Signed-off-by: Aditya Sharma <adi.sharma@zohomail.in>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261001072449.1377267-3-adi.sharma@zohomail.in

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