xfs: merge adjacent io completions of the same type

It's possible for pagecache writeback to split up a large amount of work
into smaller pieces for throttling purposes or to reduce the amount of
time a writeback operation is pending.  Whatever the reason, XFS can end
up with a bunch of IO completions that call for the same operation to be
performed on a contiguous extent mapping.  Since mappings are extent
based in XFS, we'd prefer to run fewer transactions when we can.

When we're processing an ioend on the list of io completions, check to
see if the next items on the list are both adjacent and of the same
type.  If so, we can merge the completions to reduce transaction
overhead.

On fast storage this doesn't seem to make much of a difference in
performance, though the number of transactions for an overnight xfstests
run seems to drop by ~5%.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>