Add BIO_SPEEDUP

Add BIO_SPEEDUP bio command and g_io_speedup wrapper. It tells the
lower layers that the upper layers are dealing with some shortage
(dirty pages and/or disk blocks). The lower layers should do what they
can to speed up anything that's been delayed.

The first use will be to tell the CAM I/O scheduler that any TRIM
shaping should be short-circuited because the system needs
blocks. We'll also call it when there's too many resources used by
UFS.

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