Handle API change in `rq_for_each_segment`

The API of `rq_for_each_segment` changed, most likely in Linux 3.14 due
to commit 4550dd6c6b062fc5e5b647296d55da22616123c3.

This patch adds conditional compilation to work around this and support
both API versions.
Merge pull request #65 from scality/fix-compilation-3.14

Handle API change in `rq_for_each_segment`