kern: poll: tap out the pollfd array on successful return

We do this in kern_poll() to include freebsd32 but exclude the linux
compat layer. The ABI should be the same, but the POLL constants are
probably different or should be assumed so.

Reviewed by: bapt, jhb

(cherry picked from commit b5d2165b9046572c9361a8e8b09323a6001ac586)