Update kernel-bdi-setup-and-register test for warn_unused_result attribute.

Linux 3.13 updated the backing-dev.h header to require that the return value of bdi_setup_and_register be checked, which causes the kernel-bdi-setup-and-register test to fail.

The testing I did for this fix is minimal: I verified that zfs now builds under Fedora 20 with the 3.13.3 kernel (specifically, Fedora's kernel-3.13.3-201.fc20.x86_64), and that I could mount a ZFS filesystem and access a few files.

This fixes the issue referenced in #2102.