After updating my Fedora box, I noticed that the ZFS kernel module was no longer loaded. The error (from /var/lib/dkms/zfs/0.7.12/build/make.log) was:

/var/lib/dkms/zfs/0.7.12/build/include/zpios-ctl.h:186:11: error: implicit declaration of function ‘current_kernel_time’; did you mean ‘current_kernel_time64’? [-Werror=implicit-function-declaration]        
  ts_now = current_kernel_time();                                                                                                                                                                              
/var/lib/dkms/zfs/0.7.12/build/include/zpios-ctl.h:186:9: error: incompatible types when assigning to type ‘struct timespec’ from type ‘int’                                                                   
  ts_now = current_kernel_time();  

The easiest fix is to:

Download zfs-linux_0.6.5.11-1ubuntu3.6.debian.tar.xz extract and get debian/patches/3204-Add-4.20-timespec-compat-fix.patch

Next you need to patch /var/lib/dkms/zfs/0.7.12/source/include/zpios-ctl.h with that patch

dkms autoinstall
modprobe zfs