lib: crc32: Mark test data __initconst

So it gets discarded after the selftest.

Signed-off-by: George Spelvin <linux@horizon.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Change-Id: Ie49513cbe5c1a787eaaab89c3fedcf329d51c5bd