Make sbuf_drain safe for external use

[rscheff]

Summary

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

[tuexen]

I would separate the changes:

1. Remove some constraints when sbuf_drain() is called.
2. Simplify a user of sbuf_drain().

[rscheff]

• only the subr_sbuf.c changes