xfs: reduce AGF hold times during fstrim operations

fstrim will hold the AGF lock for as long as it takes to walk and
discard all the free space in the AG that meets the userspace trim
criteria. For AGs with lots of free space extents (e.g. millions)
or the underlying device is really slow at processing discard
requests (e.g. Ceph RBD), this means the AGF hold time is often
measured in minutes to hours, not a few milliseconds as we normal
see with non-discard based operations.

This can result in the entire filesystem hanging whilst the
long-running fstrim is in progress. We can have transactions get
stuck waiting for the AGF lock (data or metadata extent allocation
and freeing), and then more transactions get stuck waiting on the
locks those transactions hold. We can get to the point where fstrim
blocks an extent allocation or free operation long enough that it
ends up pinning the tail of the log and the log then runs out of
space. At this point, every modification in the filesystem gets
blocked. This includes read operations, if atime updates need to be
made.

To fix this problem, we need to be able to discard free space
extents safely without holding the AGF lock. Fortunately, we already
do this with online discard via busy extents. We can mark free space
extents as "busy being discarded" under the AGF lock and then unlock
the AGF, knowing that nobody will be able to allocate that free
space extent until we remove it from the busy tree.

Modify xfs_trim_extents to use the same asynchronous discard
mechanism backed by busy extents as is used with online discard.
This results in the AGF only needing to be held for short periods of
time and it is never held while we issue discards. Hence if discard
submission gets throttled because it is slow and/or there are lots
of them, we aren't preventing other operations from being performed
on AGF while we wait for discards to complete...

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>