Stop creating socket aio kprocs during boot.

Create the initial pool of kprocs on demand when the first socket AIO
request is submitted instead. The pool of kprocs used for other AIO
requests is similarly created on first use.

Reviewed by: asomers
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D32468

(cherry picked from commit d1b6fef0751b70819e632d7d4722efbc8f94b80b)