snapuserd: I/O requests which are not block aligned.

If the I/O request is for a sector which is not block
aligned, then we will not find the mapping directly
to a COW op as each op is block aligned. Thus, we
handle them by chopping the I/O request and processing
the unaligned data. Furthermore, if the request
doesn't map to any of the COW ops, then route the
I/O to the base device.

Bug: 196929997
Test: Full OTA on CF - during boot up, we get an I/O request
to read superblock on system partition which is not block aligned.

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ie40633c0c6bc0c87b681c051c74c0ac787e34d9c