btrfs: set EXTENT_NORESERVE before range unlock in btrfs_truncate_block()

Set the EXTENT_NORESERVE bit in the io tree before unlocking the range so
that we can use the cached state and speedup the operation, since the
unlock operation releases the cached state.

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