powerpc: Improve pvo allocation code

Added allocation retry loop in alloc_pvo_entry(), to wait for
memory to become available if the caller specifies the M_WAITOK flag.

Also, the loop in moa64_enter() was removed, as moea64_pvo_enter()
never returns ENOMEM. It is alloc_pvo_entry() memory allocation that
can fail and must be retried.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D21035