Make sbuf_drain safe for external use

While sbuf_drain was an internal function, two
KASSERTS checked the sanity of it being called.
However, an external caller may be ignorant if
there is any data to drain, or if an error has
already accumulated. Be nice and return immediately
with the accumulated error.

MFC after: 2 weeks
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29544