sockbufs: add sbreserve_locked_limit() with custom maxsockbuf limit.

Protocols such as netlink may need a large socket receive buffer,
measured in tens of megabytes. This change allows netlink to
set larger socket buffers (given the privs are in place), without
requiring user to manuall bump maxsockbuf.

Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D36747