Remove sbmtrim().
Avoid coalescing if new mbuf is normal, but previous one is M_NOTREADY.

Sponsored by:	Nginx, Inc.
Assert that we are never asked to sbcut() an M_NOTREADY mbuf partially.

Sponsored by:	Nginx, Inc.
Rename sf_io_done() to sf_iodone(), to better match VOP_GETPAGES_ASYNC()
argument name.