proc: test lseek on /proc/net/dev

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>