Remove the block size alignment restriction.

Rather than require block-size alignment, instead bump the requested
file size to the necessary alignment. This ensures that the final block
is usable without placing onerous restrictions on the caller to figure
out the file system's block size.

This will require callers (namely, gsid) to track the actual desired
image size separately from the flie size.

This patch also updates tests to use the actual filesize of the
filesystem, rather than hardcoded 4096.

Bug: 126230649
Test: fiemap_writer_test gtest
Change-Id: I000cca274718c3ceac526d7c3392fe3a23bb42bc