genetlink: make info in GENL_REQ_ATTR_CHECK() const

Make the local variable in GENL_REQ_ATTR_CHECK() const.
genl_info_dump() returns a const pointer, so the macro
is currently hard to use in genl dumps.

Link: https://lore.kernel.org/r/20240222222819.156320-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>