core: Replace PA_PAGE_SIZE with pa_page_size()

core: Replace PA_PAGE_SIZE with pa_page_size()

PA_PAGE_SIZE using sysconf() may return a negative number

CID 1137925, CID 1137926, CID 1138485

instead of calling sysconf() directly, add function pa_page_size()
which uses the guestimate 4096 in case sysconf(_SC_PAGE_SIZE) fails

using PA_ONCE to only evaluate sysconf() once