lib: crc32: Add some additional __pure annotations

In case they help the compiler.

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: I0e0416bccd61cf737d3aac0b4f6ddd8c1af65355