gmirror: Pre-allocate the timeout event structure

We can't call malloc(M_WAITOK) in a callout handler.

Reviewed by: imp
Reported by: pho
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29223

(cherry picked from commit 2f1cfb7f63ca744e7a143896347bdc8606c291d6)