btrfs: fix btrfs_extend_item() comment

The size parameter to btrfs_extend_item() is the number of bytes
to add to the item, not the size of the item after the operation
(like it is for btrfs_truncate_item(), there the size parameter
is not the number of bytes to take away, but the total size of
the item after truncation).
Fix it in the comment.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>