xfs: set firstfsb to NULLFSBLOCK before feeding it to _bmapi_write

We must initialize the firstfsb parameter to _bmapi_write so that it
doesn't incorrectly treat stack garbage as a restriction on which AGs
it can search for free space.

Fixes-coverity-id: 1402025
Fixes-coverity-id: 1415167
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NBrian Foster <bfoster@redhat.com>

fs/xfs/libxfs/xfs_bmap.c

@@ -6499,6 +6499,15 @@ xfs_bmap_finish_one(
 	xfs_fsblock_t			firstfsb;
 	int				error = 0;
 
+	/*
+	 * firstfsb is tied to the transaction lifetime and is used to
+	 * ensure correct AG locking order and schedule work item
+	 * continuations.  XFS_BUI_MAX_FAST_EXTENTS (== 1) restricts us
+	 * to only making one bmap call per transaction, so it should
+	 * be safe to have it as a local variable here.
+	 */
+	firstfsb = NULLFSBLOCK;
+
 	trace_xfs_bmap_deferred(tp->t_mountp,
 			XFS_FSB_TO_AGNO(tp->t_mountp, startblock), type,
 			XFS_FSB_TO_AGBNO(tp->t_mountp, startblock),

fs/xfs/xfs_reflink.c

@@ -329,7 +329,7 @@ xfs_reflink_convert_cow_extent(
 	xfs_filblks_t			count_fsb,
 	struct xfs_defer_ops		*dfops)
 {
-	xfs_fsblock_t			first_block;
+	xfs_fsblock_t			first_block = NULLFSBLOCK;
 	int				nimaps = 1;
 
 	if (imap->br_state == XFS_EXT_NORM)