common: silence GCC warnings

common: silence GCC warnings

Preforker.h:111:8: warning: ignoring return value of ‘ssize_t
safe_write(int, const void*, size_t)’, declared with attribute
warn_unused_result [-Wunused-result]
        (void)safe_write(fd[1], &r, sizeof(r));
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and

compat.cc:36:28: warning: comparison between signed and unsigned integer
expressions [-Wsign-compare]
     if (off + sizeof(data) > len)
         ~~~~~~~~~~~~~~~~~~~^~~~~

Fixes: http://tracker.ceph.com/issues/23774
Signed-off-by: Kefu Chai <kchai@redhat.com>