compat: avoid unused warn with TEMP_FAILURE_RETRY

The version of TEMP_FAILURE_RETRY found on Linux has a GNU extension
that squashes the unused return value warning where applicable. This
adds a VOID_TEMP_FAILURE_RETRY to make the case explicit, casting the
expression value to void to avoid the warning.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #1303 from ceph/port/temp-failure-retry

compat: avoid unused warn with TEMP_FAILURE_RETRY

Reviewed-by: Sage Weil <sage@inktank.com>