Revert "Properly ignore bdi_setup_and_register return value"

Older GCC versions do not obey -Wno-unused-result. This reverts commit
729210564a5325e190fc4fba22bf17bacf957ace in favor of a solution that
does not require -Wno-unused-result.
Linux 3.13 compat: Handle __must_check bdi_setup_and_register

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