netpoll: rename and export netpoll_zap_completion_queue()

netpoll: rename and export netpoll_zap_completion_queue()

zap_completion_queue() drains the per-CPU softnet completion queue.
Rename it with the netpoll_ prefix shared by the rest of the
subsystem's public API, and promote it from file-static to
EXPORT_SYMBOL_NS_GPL in the NETDEV_INTERNAL namespace so the upcoming
netconsole-side find_skb() can call it once the function moves out.
A forward declaration is added to include/linux/netpoll.h, and the
old file-static forward declaration is dropped.

No functional change.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260512-netconsole_split-v2-8-1191d14ad66d@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>