blk: pad flog entries and deal with map entries in CL-sizes

Also replace spin locks with mutexes since userspace implementation performs better that way.
To be honest, I don't think this really solves the issue. The applied mask results in a multiple of 64, so only 4 locks will be used one way or the other. I was thinking of something along the lines of:

int map_lock_num = (premap_lba / (BTT_MAP_LOCK_ALIGN / sizeof(mapentry))) % bttp->nfree;

This would map lbas 0..15 -> lock 0, 16..31 -> 1 and so on, but there is a downside in the added divisions...