netpoll: extract IPv4 address retrieval into helper function

Move the IPv4 address retrieval logic from netpoll_setup() into a
separate netpoll_take_ipv4() function to improve code organization
and readability. This change consolidates the IPv4-specific logic
and error handling into a dedicated function while maintaining
the same functionality.

No functional changes.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250618-netpoll_ip_ref-v1-2-c2ac00fe558f@debian.org


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