Add VERIFY0P() and ASSERT0P() macros.

These macros are similar to VERIFY0() and ASSERT0() but are intended
for pointers, and therefore use uintptr_t instead of int64_t.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Dag-Erling Smørgrav <des@FreeBSD.org>
Closes #15225