sockbuf: provide sbunreserve_locked() which is a complement to sbreserve()

The sbreserve() works only on protocol-independent parts of the sockbuf,
but sbrelease() also clears the generic sockbuf mbuf chain. Calling the
latter to undo changes done by the former is not correct. The new
function is the right thing.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49364