Export sbuf_drain to orchestrate lock and drain action

While exporting large amounts of data to a sysctl
request, datastructures may need to be locked.

Exporting the sbuf_drain function allows the
coordination between drain events and held
locks, to avoid stalls.

PR: 254333
Reviewed By: jhb
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29481

(cherry picked from commit c804c8f2c58ba42d476de07fbceff9ac4dd95f0e)