powerpc: slb_alloc_user_cache: fix missing uma_zalloc wait flag

Flags should be M_WAITOK | M_ZERO instead of just M_ZERO

Reviewed by: markj
MFC after: 2 days
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D36865

(cherry picked from commit ff80ab1cebd61d08598fd248fb6e70782ba72e4a)