zfs: Add upstream patch bc151f7 to fix kernel 3.18 builds

Additionally, recognize that the contents of the nix-build.patch are
included in the upstream tree as 3cd33ff and will be released in 0.6.4.
Merge pull request #4932 from wkennington/master.zfs

zfs: Add upstream patch bc151f7 to fix kernel 3.18 builds