clear_page() would normally live in asm/page.h but adding the file
and fixing the dependencies would be too much for a single line.
Move the function to the end of the file with a clear separator
and make it clear that it does not operate on a 'struct page'
but on a page address by changing the argument name and leaving
a comment.
The function is currently used by at least mthca(4) as the only
in-tree consumer, and drm-kmod ttm_pool.c.
No functional changes.
Sponsored by: The FreeBSD Foundation
MFC after: 3 days