net: netpoll: remove unused netpoll pointer from netpoll_info

net: netpoll: remove unused netpoll pointer from netpoll_info

The netpoll_info structure contains an useless pointer back to its
associated netpoll. This field is never used, and the assignment in
__netpoll_setup() is does not comtemplate multiple instances, as
reported by Jay[1].

Drop both the member and its initialization to simplify the structure.

Link: https://lore.kernel.org/all/2930648.1757463506@famine/
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250918-netpoll_jv-v1-1-67d50eeb2c26@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>