xfs: add tracepoints for each of the externally visible allocators

There are now five separate space allocator interfaces exposed to the
rest of XFS for five different strategies to find space.  Add
tracepoints for each of them so that I can tell from a trace dump
exactly which ones got called and what happened underneath them.  Add a
sixth so it's more obvious if an allocation actually happened.

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