bpf: Allow bpf_d_path() from filp_close_sync()

The d_path selftest closes its descriptors with close_range() so that
its fentry program on filp_close() runs.

Commit 056069eb2b66 ("fs: make close_range() synchronous") switched
close_range() to filp_close_sync(), so the program still attaches but
is never called:

  test_d_path_basic:FAIL:close trampoline for filp_close was not called

With that series, close(), close_range() and the closes on exec and
exit all go through filp_close_sync(). Few paths still reach
filp_close(), dup2() being one of them, so a program on filp_close()
no longer sees most file closes.

Allow bpf_d_path() from filp_close_sync(), but only from fentry.
filp_close_sync() drops the last file reference inline, so by the time
an fexit program runs, file->f_path may already be freed, with no RCU
delay for NORCU dentries such as pipes and sockets.

Attach the selftest program to filp_close_sync() and keep triggering
it with close_range().

Add a test rejecting bpf_d_path() from fexit/filp_close_sync.

Fixes: 056069eb2b66 ("fs: make close_range() synchronous")
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://patch.msgid.link/20261021201400.5409-1-ihor.solodrai@linux.dev
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>