xfs: make sure aglen never goes negative in xfs_refcount_adjust_extents

Prior to calling xfs_refcount_adjust_extents, we trimmed agbno/aglen
such that the end of the range would not be in the middle of a refcount
record.  If this is no longer the case, something is seriously wrong
with the btree.  Bail out with a corruption error.

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