sendfile_test: fix copy-paste bug

Require the newly opened file descriptor to be good, instead of
re-requiring the one that was required three lines earlier.
Thankfully, opening /dev/null is really unlikely to fail.

Reported by: Coverity
MFC after: 1 week
Sponsored by: Dell EMC Isilon

(cherry picked from commit a8fea07c3053293f35e00b9073b33ff6f1d9a067)