mm/nommu.c: Switch __get_user_pages_unlocked() to use __get_user_pages()

Extracted from commit cde70140fed8 "mm/gup: Overload get_user_pages()
functions".  This is needed before picking commit 768ae309a961
"mm: replace get_user_pages() write/force parameters with gup_flags".

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>