There's a merged pull request on the void-packages repository

zfs: support building against linux5.6
https://github.com/void-linux/void-packages/pull/20506

Description:
This revision backports a patch to ZFS 0.8.3 to support building on Linux 5.6, which introduced some new interfaces used by the module and removed some deprecated functions.

Rather than wait for upstream to push out another release (which seems to take awhile), I figure it's best to get the pending solution out.

I've confirmed that the DKMS module builds successfully on linux{4.9,5.2,5.3,5.4,5.5,5.6} on x86_64; I have no other architectures on which to test.

From https://github.com/openzfs/zfs/pull/9961