Btrfs: return error code in btrfs_check_trunc_cache_free_space()

Fix to return error code instead always return 0 from function
btrfs_check_trunc_cache_free_space().
Introduced by commit 7b61cd92242542944fc27024900c495a6a7b3396
(Btrfs: don't use global block reservation for inode cache truncation)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>