xfs: only allow reaping of per-AG blocks in xrep_reap_extents

Now that we've refactored btree cursors to require the caller to pass in
a perag structure, there are numerous problems in xrep_reap_extents if
it's being called to reap extents for an inode metadata repair.  We
don't have any repair functions that can do that, so drop the support
for now.

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