Block allocation requires a permanent transaction for deferred AGFL
frees.  Add an assert in the block allocation path to make explicit and
obvious to future callers the requirement of a transaction with a
permanent reservation.
Reported-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NBrian Foster <bfoster@redhat.com>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
[darrick: split this out from the previous patch per hch request]
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>