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

(cherry picked from commit 7b660faa9e30c15d3be9b2c44c3ca046a33331f4)