sbuf(9): Microoptimize sbuf_put_byte()

This function is actively used by sbuf_vprintf(), so this simple
inlining in half reduces time of kern.geom.confxml generation.

MFC after: 2 weeks
Sponsored by: iXsystem, Inc.

(cherry picked from commit 7835b2cb4a1ae57f403739a2f1076ec7188f18c9)