genetlink: extend info user-storage to match NL cb ctx

genetlink: extend info user-storage to match NL cb ctx

This allows a more uniform implementation of non-dump and dump
operations, and will be used later in the series to avoid some
per-operation allocation.

Additionally rename the NL_ASSERT_DUMP_CTX_FITS macro, to
fit a more extended usage.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/1130cc2896626b84587a2a5f96a5c6829638f4da.1728460186.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>