new testcase: kmsg01

Test /dev/kmsg based on kernel doc: Documentation/ABI/testing/dev-kmsg
 - read() blocks
 - non-blocking read() fails with EAGAIN
 - partial read fails (buffer smaller than message)
 - can write to /dev/kmsg and message seqno grows
 - first read() after open() returns same message
 - if messages get overwritten, read() returns -EPIPE
 - device supports SEEK_SET, SEEK_END, SEEK_DATA

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Cyril Hrubis <chrubis@suse.cz>