[AF_KEY]: suppress a warning for 64k pages.

On PowerPC allmodconfig build we get this:

net/key/af_key.c:400: warning: comparison is always false due to limited range of data type

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>