Linux 3.13 compat: Handle __must_check bdi_setup_and_register

torvalds/linux@8077c0d983ab276ec5f2700df56a64d671781905 added a
__must_check to the bdi_setup_and_register(), which caused our autotools
check to break. zfsonlinux/zfs@729210564a5325e190fc4fba22bf17bacf957ace
was intended to correct that, but it depended on -Wno-unused-result,
which is unrecognized in older GCC versions. That commit has been
reverted in favor of a solution that does not require
-Wno-unused-result.

Closes zfsonlinux/zfs#2102
Closes zfsonlinux/zfs#2135

Signed-off-by: Richard Yao <ryao@gentoo.org>