erasure-code: tests must use aligned buffers

The underlying code assumes the memory buffer is aligned on a long
boundary which is not always the case. Using buffer::create_page_aligned
which calls posix_memalign ensure the allocated buffer starts at an
address that is properly aligned.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #961 from dachary/wip-erasure-code-alignment

erasure-code: tests must use aligned buffers

Reviewed-by: Sage Weil <sage@inktank.com>