Btrfs: add an assert to btrfs_lookup_csums_range for alignment

I was hitting weird issues when trying to remove hole extents and it turned out
it was because I was sending non-aligned offsets down to
btrfs_lookup_csums_range.  So add an assert for this in case somebody trips over
this in the future.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/file-item.c

@@ -329,6 +329,9 @@ int btrfs_lookup_csums_range(struct btrfs_root *root, u64 start, u64 end,
 	u64 csum_end;
 	u16 csum_size = btrfs_super_csum_size(root->fs_info->super_copy);
 
+	ASSERT(start == ALIGN(start, root->sectorsize) &&
+	       (end + 1) == ALIGN(end + 1, root->sectorsize));
+
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;