Out of line get_user_pages_fast fallback implementation, make it a weak
symbol, get rid of CONFIG_HAVE_GET_USER_PAGES_FAST.

Export the symbol to modules so lguest can use it.
Signed-off-by: NNick Piggin <npiggin@suse.de>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>