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.
Thanks, Tomasz.  I see what you mean.  I think a good fix here would be to use map_entry_off instead of premap_lba.  Just a couple lines higher, we calculate the byte offset of the map entry so if we round that down to a cache line, and then take the modulo, I think it fixes this issue:

`int map_lock_num = (map_entry_off & ~(BTT_MAP_LOCK_ALIGN - 1)) % bttp->nfree;`