xfs: improve the code that checks recovered extent-free intent items

The code that validates recovered extent-free intent items is kind of a
mess -- it doesn't use the standard xfs type validators, and it doesn't
check for things that it should.  Fix the validator function to use the
standard validation helpers and look for more types of obvious errors.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/xfs_extfree_item.c

@@ -584,14 +584,13 @@ xfs_efi_validate_ext(
 	struct xfs_mount		*mp,
 	struct xfs_extent		*extp)
 {
-	xfs_fsblock_t			startblock_fsb;
+	if (extp->ext_start + extp->ext_len <= extp->ext_start)
+		return false;
 
-	startblock_fsb = XFS_BB_TO_FSB(mp,
-			   XFS_FSB_TO_DADDR(mp, extp->ext_start));
-	if (startblock_fsb == 0 ||
-	    extp->ext_len == 0 ||
-	    startblock_fsb >= mp->m_sb.sb_dblocks ||
-	    extp->ext_len >= mp->m_sb.sb_agblocks)
+	if (!xfs_verify_fsbno(mp, extp->ext_start))
+		return false;
+
+	if (!xfs_verify_fsbno(mp, extp->ext_start + extp->ext_len - 1))
 		return false;
 
 	return true;