linux/configure: always use our own split_page
linux/configure: always use our own split_page

split_page() is always defined, but it may be exported
or not. Detecting the export is very hard to do in a
reliable way, therefore we always use our own.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap