Calculate mirror log size instead of hardcoding 1 extent size.

Calculate mirror log size instead of hardcoding 1 extent size.

It fails for 1k PE now.

Patch adds log_region_size into allocation habdle struct
and use it in _alloc_parallel_area() for proper log size calculation
instead of hardcoded 1 extent - which can fail.

Reproducer for incorrect log size calculation:
        DEV=/dev/sd[bcd]

        pvcreate $DEV
        vgcreate -s 1k vg_test $DEV
        lvcreate -m1 -L 12M -n mirr vg_test

https://bugzilla.redhat.com/show_bug.cgi?id=477040

The log size calculation is mostly copied from kernel code.