arm64: Fix an assertion in pmap_enter_largepage()

The intent is to assert that either no mapping exists at the given VA,
or that the existing L1 block mapping maps the same PA.

Fixes: 36f1526a598c ("Add experimental 16k page support on arm64")
Reviewed by: alc
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36698