liblp: Reclaim wasted space from unaligned partitions.

When allocating a partition with a size that is unaligned (to the
optimal alignment), the remaining sectors are wasted since they are
never reallocated. This is because the free list is guaranteed to only
contain optimally-aligned regions. Unfortunately this means when a
partition is resized, we are wasting a small amount of space each time.
On a non-A/B device, this could wind up being significant.

For example, with an alignment of 512KiB, a 4KiB partition at offset 0
will waste 508KiB of space. The next extent to be allocated by any
partition will start at the next 512KiB.

To address this, we round up extents to the optimal alignment. This
means partitions may wind up slightly over-allocated, versus before,
where they would waste space by making it unavailable.

Bug: 120434950
Test: liblp_test gtest
Change-Id: I555209b301058555526cc4309f7049ae81cf877d