ErasureCode: coding style conformance

Signed-off-by: Loic Dachary <loic@dachary.org>
ErasureCode: optimize padding calculation

Andreas-Joachim Peters pointed out that the ErasureCodeJerasure method
pad_in_length is very inefficient for no reason. It is replaced by the
alignment method and the calculation is inlined in the encode method.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #619 from dachary/wip-erasure-pad

ErasureCode: optimize padding calculation 

Reviewed-by: Sage Weil <sage@inktank.com>