ethtool: rss: factor out allocating memory for response

ethtool: rss: factor out allocating memory for response

To ease the code reuse for RSS_CREATE we'll want to prepare
struct rss_reply_data for the new context. Unfortunately
we can't depend on the exiting scaffolding because the context
doesn't exist (ctx=NULL) when we start preparing. Factor out
the portion of the context 0 handling responsible for allocation
of request memory, so that we can call it directly.

Link: https://patch.msgid.link/20250717234343.2328602-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>