macsec: always read MACSEC_SA_ATTR_PN as a u64

Currently, MACSEC_SA_ATTR_PN is handled inconsistently, sometimes as a
u32, sometimes forced into a u64 without checking the actual length of
the attribute. Instead, we can use nla_get_u64 everywhere, which will
read up to 64 bits into a u64, capped by the actual length of the
attribute coming from userspace.

This fixes several issues:
 - the check in validate_add_rxsa doesn't work with 32-bit attributes
 - the checks in validate_add_txsa and validate_upd_sa incorrectly
   reject X << 32 (with X != 0)

Fixes: 48ef50fa

 ("macsec: Netlink support of XPN cipher suites (IEEE 802.1AEbw)")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>