pfsync: Allocate and initialize buckets before attaching the interface

This prevents a potential race that the ioctl threads see NULL or
uninitialized buckets.

Reviewed by: kp
Fixes: 4fc65bcbe3fb pfsync: Performance improvement
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51064

(cherry picked from commit edc307eca9a9a9b0ce7445cff513b48f6489e5c6)