proc: test lseek on /proc/net/dev

This line in tools/testing/selftests/proc/read.c was added to catch
oopses, not to verify lseek correctness:

        (void)lseek(fd, 0, SEEK_SET);

Oh, well. Prevent more embarassement with simple test.

Link: https://lkml.kernel.org/r/aKTCfMuRXOpjBXxI@p183
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/proc/.gitignore

@@ -7,6 +7,7 @@
 /proc-loadavg-001
 /proc-maps-race
 /proc-multiple-procfs
+/proc-net-dev-lseek
 /proc-empty-vm
 /proc-pid-vm
 /proc-self-map-files-001