[PPC64] Fix wrong KASSERT in mphyp_pte_insert()

As mphyp_pte_unset() can also remove PTE entries, and as this can
happen in parallel with PTEs evicted by mphyp_pte_insert(), there
is a (rare) chance the PTE being evicted gets removed before
mphyp_pte_insert() is able to do so. Thus, the KASSERT should
check wether the result is H_SUCCESS or H_NOT_FOUND, to avoid
panics if the situation described above occurs.

More details about this issue can be found in PR 237470.

PR: 237470
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D20012