net: ethtool: dynamically allocate full req size req

In preparation for using req_info to carry parameters between
SET and NTF allocate a full request info struct. Since the size
depends on the subcommand we need to allocate it on the heap.

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250623231720.3124717-3-kuba@kernel.org


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