xfs: add a tracepoint for blockgc scans

Add some tracepoints so that we can observe when the speculative
preallocation garbage collector runs.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
3 files changed