snapuserd: Address alignment fault on 32-bit systems

When the scratch space is mmap'ed, the metadata buffer
will be un-aligned. This may lead to alignment fault
on 32-bit systems. Address this by temporarily copying
it to buffer.

No perf impact as this code path is not in I/O path
and the copy is a for the size of metadata buffer which
is 8k.

Bug: 206426215
Test: Full and Incremental OTA on pixel
1: Compile snapuserd as 32 bit and reproduced the bug on pixel.
2: With fix - OTA applied successfully.
3: Reboot the device when merge was in-flight as the fix is primarily
in that path.
4: Verify merge completion and data integrity post merge.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Icd4a21d6a61f1ab36e65994c06a4d049a2ee741c
Merged-In: I63c0d862057ebf138c9d1696a942030e30598739