btrfs: use the btrfs_extent_map_end() helper everywhere

btrfs: use the btrfs_extent_map_end() helper everywhere

We have a helper to calculate an extent map's exclusive end offset, but
we only use it in some places. Update every site that open codes the
calculation to use the helper.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>