xfs: also truncate holes covered by COW blocks

This only matters if we want to write data through the COW fork that is
not actually an overwrite of existing data.  Reasons for that are
speculative COW fork allocations using the cowextsize, or a mode where
we always write through the COW fork.  Currently both can't actually
happen, but I plan to enable them.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_aops.c

@@ -447,27 +447,28 @@ xfs_map_blocks(
 
 	wpc->fork = XFS_DATA_FORK;
 
+	/* landed in a hole or beyond EOF? */
 	if (imap.br_startoff > offset_fsb) {
-		/* landed in a hole or beyond EOF */
 		imap.br_blockcount = imap.br_startoff - offset_fsb;
 		imap.br_startoff = offset_fsb;
 		imap.br_startblock = HOLESTARTBLOCK;
 		imap.br_state = XFS_EXT_NORM;
-	} else {
+	}
+
 	/*
-		 * Truncate to the next COW extent if there is one.  This is the
-		 * only opportunity to do this because we can skip COW fork
-		 * lookups for the subsequent blocks in the mapping; however,
-		 * the requirement to treat the COW range separately remains.
+	 * Truncate to the next COW extent if there is one.  This is the only
+	 * opportunity to do this because we can skip COW fork lookups for the
+	 * subsequent blocks in the mapping; however, the requirement to treat
+	 * the COW range separately remains.
 	 */
 	if (cow_fsb != NULLFILEOFF &&
 	    cow_fsb < imap.br_startoff + imap.br_blockcount)
 		imap.br_blockcount = cow_fsb - imap.br_startoff;
 
 	/* got a delalloc extent? */
-		if (isnullstartblock(imap.br_startblock))
+	if (imap.br_startblock != HOLESTARTBLOCK &&
+	    isnullstartblock(imap.br_startblock))
 		goto allocate_blocks;
-	}
 
 	wpc->imap = imap;
 	trace_xfs_map_blocks_found(ip, offset, count, wpc->fork, &imap);