netdev: Add methods to do netdev-specific argument comparisons.

netdev: Add methods to do netdev-specific argument comparisons.

When doing a netdev_open(), a check is first done to make sure the
arguments are equivalent for any open devices with the same name.  In
most cases, a simple shash comparison is sufficient.  However, IPsec
key configuration is handled by an external program, so it is not pushed
down into the kernel module.  Thus, when the "unparse_config" method is
called on an existing IPsec-based vport, a simple comparison with the
returned data will not match the original configuration.  This commit
adds code to allow netdev-specific argument comparisons and has
"ipsec_gre" make use of them.

Bug #5575