lustre: Convert ll_get_user_pages() to use get_user_pages_fast()

CC: Peng Tao <tao.peng@emc.com>
Acked-by: Andreas Dilger <andreas.dilger@intel.com>
CC: hpdd-discuss@lists.01.org
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>