[PATCH] do_wp_page: cannot share file page

A small optimization to do_wp_page's check for whether to avoid copy by
reusing the page already mapped.  It can never share a cached file page,
nor can it share a reserved page (often the empty zero page), so it's a
waste of time to lock and unlock in those cases.  Which nowadays can both
be neatly excluded by a preliminary PageAnon test.

Christoph has reported that a preliminary page_count test proved valuable
for scalability here, but PageAnon covers more common cases all at once.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>