[PATCH 0/4] improved busy extent handling V5

[Christoph Hellwig]

This series optimizes how XFS deals with busy extents.  It starts to
track them exactly, and allows reuses where possible (metadata to metadata)
or else tries to avoid busy extents during allocations.  This means
we don't have a single log force due to busy extents during either
xfstests, compilebench or postmark on my testsystem, which can easily
be tracked using the new tracepoints added in the last patch.

Changes from V4:
 - fix xfs_alloc_busy_trim tracing
Changes from V3:
 - address review comments from Alex in patch 2
 - add a new patch to reduce pagb_lock roundtrips
Changes from V2:
 - remove the bmapi userdata tracking now that is unused
 - restructure the busy extent reuse code to make it more suitable
   for the discard support
 - reshuffle the patch boundaries