erasure-code/isa: don't use typename outside of template

error: erasure-code/isa/ErasureCodeIsa.h:141: using 'typename' outside of template

Signed-off-by: Sage Weil <sage@redhat.com>
erasure-code/isa: fix signed/unsigned comparison

warning: erasure-code/isa/ErasureCodeIsa.cc:313: comparison between signed and unsigned integer expressions

Signed-off-by: Sage Weil <sage@redhat.com>