Make sendfile_file test conditional

For certain kernel versions, the test fails with OSError: [EINVAL]

According to man(2) sendfile:

In Linux 2.4 and earlier, out_fd could also refer to a regular file; this
possibility went away in the Linux 2.6.x kernel series, but was restored in
Linux 2.6.33

This patch skips the test sendfile_file on kernel versions which fall in the
above range