vm: Initialize the transient buffer mapping arena with M_WAITOK

The wait flag is passed to UMA when allocating boundary tags for the
initial span, and UMA expects either M_WAITOK or M_NOWAIT to be present.

Reported by: cperciva
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f82177b8cfd2269aed060fe72e672ad4115ec8c3)