xfs: assert that we don't enter agfl freeing with a non-permanent transaction

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: Darrick J. Wong <[e-mail]>
Signed-off-by: Brian Foster <[e-mail]>
Reviewed-by: Darrick J. Wong <[e-mail]>
[darrick: split this out from the previous patch per hch request]
Signed-off-by: Darrick J. Wong <[e-mail]>

fs/xfs/libxfs/xfs_alloc.c

···
 	xfs_extlen_t		need;	/* total blocks needed in freelist */
 	int			error = 0;
 
+	/* deferred ops (AGFL block frees) require permanent transactions */
+	ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
+
 	if (!pag->pagf_init) {
 		error = xfs_alloc_read_agf(mp, tp, args->agno, flags, &agbp);
 		if (error)