[IPSEC]: Make xfrm_lookup flags argument a bit-field

This patch introduces an enum for bits in the flags argument of xfrm_lookup.
This is so that we can cram more information into it later.

Since all current users use just the values 0 and 1, XFRM_LOOKUP_WAIT has
been added with the value 1 << 0 to represent the current meaning of flags.

The test in __xfrm_lookup has been changed accordingly.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>