buf: Dynamically allocate per-CPU buffer queues

To reduce static bloat. No functional change intended.

PR: 269572
Reviewed by: mjg, kib, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39808

(cherry picked from commit e72f7ed43eefaf305c33c232bc2c33d997427f58)