powerpc64/mmu: Make moea64_pvo_enter() return if an entry already exists

Summary:
Instead of searching for a PVO entry before adding, take advantage of
the fact that RB_INSERT() returns NULL if it inserts, and the existing entry if
an entry exists, without inserting a new entry. This saves an extra tree
traversal in the cases where the PVO does not exist.

Reviewed by: luporl
Differential Revision: https://reviews.freebsd.org/D20944