netlink: specs: support setting prefix-name per attribute

Ethtool's PSE PoDL has a attr nest with different prefixes:

/* Power Sourcing Equipment */
enum {
	ETHTOOL_A_PSE_UNSPEC,
	ETHTOOL_A_PSE_HEADER,			/* nest - _A_HEADER_* */
	ETHTOOL_A_PODL_PSE_ADMIN_STATE,		/* u32 */
	ETHTOOL_A_PODL_PSE_ADMIN_CONTROL,	/* u32 */
	ETHTOOL_A_PODL_PSE_PW_D_STATUS,		/* u32 */

Header has a prefix of ETHTOOL_A_PSE_ and other attrs prefix of
ETHTOOL_A_PODL_PSE_ we can't cover them uniformly.
If PODL was after PSE life would be easy.

Now we either need to add prefixes to attr names which is yucky
or support setting prefix name per attr.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>