zfs: Fix libblkid detection (backport from upstream)

This fixes issues with importing ZFS pools, such as when the ZFS device is a
partition that covers the end of the disk.

See: https://github.com/zfsonlinux/zfs/issues/1684
zfs: Don't look for devices only in /dev

If we don't give out a directory to 'zpool import', it will use libblkid
to automatically find all existing ZFS devices.
zfs: Fix minor filesystem corruption with gcc 4.8

It turns out that some of gcc 4.8's aggressive optimizations may
cause minor filesystem corruption in ZFS. To fix it, a patch was
cherry-picked from the upstream's git tree.

See: https://github.com/zfsonlinux/zfs/pull/2051
spl: Fix compilation on 3.12 and later kernels
spl: Fix compilation on 3.13 and later kernels
zfs: Fix compilation on 3.13 and later kernels
Merge branch 'u/zfs-import' of git://github.com/wizeman/nixpkgs

zfs: Misc fixes