arm64_pmap: narrow scope of bti_same test

The pmap_bti_same test in pmap_enter_l3c only happens in the
!ADDR_IS_KERNEL case; in the other case, a KASSERT fails. So move the
test into that case to save a bit of time when ADDR_IS_KERNEL.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D45160