fs: erofs: add unaligned read range handling

I'm not an expert on erofs, but my quick glance didn't expose any
special handling on unaligned range, thus I think the U-boot erofs
driver doesn't really support unaligned read range.

This patch will add erofs_get_blocksize() so erofs can benefit from the
generic unaligned read support.

Cc: Huang Jianan <jnhuang95@gmail.com>
Cc: linux-erofs@lists.ozlabs.org
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Huang Jianan <jnhuang95@gmail.com>