Btrfs: adjust len of writes if following a preallocated extent

If we have

|0--hole--4095||4096--preallocate--12287|

instead of using preallocated space, a 8K direct write will just
create a new 8K extent and it'll end up with

|0--new extent--8191||8192--preallocate--12287|

It's because we find a hole em and then go to create a new 8K
extent directly without adjusting @len.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Chris Mason <clm@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -7782,10 +7782,12 @@ static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
 	}
 
 	/*
-	 * this will cow the extent, reset the len in case we changed
-	 * it above
+	 * this will cow the extent, if em is within [start, len], then
+	 * probably we've found a preallocated/existing extent, let's
+	 * give it a chance to use preallocated space.
 	 */
-	len = bh_result->b_size;
+	len = min_t(u64, bh_result->b_size, em->len - (start - em->start));
+	len = ALIGN(len, root->sectorsize);
 	free_extent_map(em);
 	em = btrfs_new_extent_direct(inode, start, len);
 	if (IS_ERR(em)) {