Implement bio_speedup

React to the BIO_SPEED command in the cam io scheduler by completing
as successful BIO_DELETE commands that are pending, up to the length
passed down in the BIO_SPEEDUP cmomand. The length passed down is a
hint for how much space on the drive needs to be recovered. By
completing the BIO_DELETE comomands, this allows the upper layers to
allocate and write to the blocks that were about to be trimmed. Since
FreeBSD implements TRIMSs as advisory, we can eliminliminate them and
go directly to writing.

The biggest benefit from TRIMS coomes ffrom the drive being able t
ooptimize its free block pool inthe log run. There's little nto no
bene3efit in the shoort term. , sepeciall whn the trim is followed by
a write. Speedup lets us make this tradeoff.

Reviewed by: kirk, kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D18351