cam: allocate CCBs from UMA for SCSI and ATA IO

This patch makes it possible for CAM to use small CCBs allocated
from an periph-specific UMA zone instead of the usual, huge ones.
The end result is that CCBs issued via da(4) take 544B (size of
ccb_scsiio) instead of the usual 2kB (size of 'union ccb', ~1.5kB,
rounded up by malloc(9)). For ATA it's 272B. We waste less
memory, we avoid zeroing the unused 1kB, and it should be easier
to allocate those CCBs in low memory conditions. It should also
be possible to use uma_zone_reserve(9) to improve behaviour
in low memory conditions even further.

Note that this does not change the size, or the layout, of CCBs
as such. CCBs get allocated in various different ways, in particular
on the stack, and I don't want to redo all that. Instead, this
provides an opt-in mechanism for the periph to declare "my start()
callback is fine with receiving a CCB allocated from this UMA zone".
In other words, most of the code works exactly as it used to; the
change only happens to IOs issued by xpt_run_allockq(), which
is - conveniently - pretty much all that matters for performance.

The reason for doing it this way is that it's pretty small, localized
change, and can be implemented gradually and iteratively: take a
periph, make sure its start() callback only casts the CCBs it takes
to a particular type of CCB, for example ccb_scsiio, and that it only
casts CCBs returned by cam_periph_getccb() to that type, then add UMA
zone for that size, and declare it safe to XPT.

This is disabled by default. Set 'kern.cam.ada.enable_uma_ccbs=1'
and 'kern.cam.da.enable_uma_ccbs=1' tunables to enable it. Testing
is welcome; I will flip the default to enable in two weeks from now.

Reviewed By: imp
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D28674