[IPSEC] xfrm_user: Kill PAGE_SIZE check in verify_sec_ctx_len()

First, it warns when PAGE_SIZE >= 64K because the ctx_len
field is 16-bits.

Secondly, if there are any real length limitations it can
be verified by the security layer security_xfrm_state_alloc()
call.

Signed-off-by: David S. Miller <davem@davemloft.net>