Add consolodation of TRIM / BIO_DELETE commands to the UFS/FFS filesystem.

When deleting files on filesystems that are stored on flash-memory
(solid-state) disk drives, the filesystem notifies the underlying
disk of the blocks that it is no longer using. The notification
allows the drive to avoid saving these blocks when it needs to
flash (zero out) one of its flash pages. These notifications of
no-longer-being-used blocks are referred to as TRIM notifications.
In FreeBSD these TRIM notifications are sent from the filesystem
to the drive using the BIO_DELETE command.

Until now, the filesystem would send a separate message to the drive
for each block of the file that was deleted. Each Gigabyte of file
size resulted in over 3000 TRIM messages being sent to the drive.
This burst of messages can overwhelm the drive's task queue causing
multiple second delays for read and write requests.

This implementation collects runs of contiguous blocks in the file
and then consolodates them into a single BIO_DELETE command to the
drive. The BIO_DELETE command describes the run of blocks as a
single large block being deleted. Each Gigabyte of file size can
result in as few as two BIO_DELETE commands and is typically less
than ten. Though these larger BIO_DELETE commands take longer to
run, they do not clog the drive task queue, so read and write
commands can intersperse effectively with them.

Though this new feature has been throughly reviewed and tested, it
is being added disabled by default so as to minimize the possibility
of disrupting the upcoming 12.0 release. It can be enabled by running
``sysctl vfs.ffs.dotrimcons=1''. Users are encouraged to test it.
If no problems arise, we will consider requesting that it be enabled
by default for 12.0.

Reviewed by: kib
Tested by: Peter Holm
Sponsored by: Netflix