bug#45401: ZFS package not compiling on recent kernel

[raid5atemyhomework]

Installing `zfs` results in the following errors:

```
In file included from 
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/include/sys/dmu.h:848:0,
                 from 
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/include/sys/spa.h:44,
                 from 
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/include/sys/spa_impl.h:35,
                 from 
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/module/zfs/vdev_disk.c:30:
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/module/zfs/vdev_disk.c: In function 
'vdev_disk_open':
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/include/linux/blkdev_compat.h:378:37: 
error: implicit declaration of function 'check_disk_change'; did you mean 
'bdev_disk_changed'? [-Werror=implicit-function-declaration]
 #define vdev_bdev_reread_part(bdev) check_disk_change(bdev)
                                     ^
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/module/zfs/vdev_disk.c:218:17: note: 
in expansion of macro 'vdev_bdev_reread_part'
     int error = vdev_bdev_reread_part(bdev);
                 ^~~~~~~~~~~~~~~~~~~~~
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/module/zfs/vdev_disk.c: In function 
'vdev_blkg_tryget':
/tmp/guix-build-zfs-0.8.5.drv-0/zfs-0.8.5/module/zfs/vdev_disk.c:506:37: error: 
'struct percpu_ref' has no member named 'count'
   rc = atomic_long_inc_not_zero(&ref->count);
                                     ^~
cc1: some warnings being treated as errors
```

This is using kernel 5.10.2 I believe:

```
downloading from 
https://ci.guix.gnu.org/nar/lzip/si6ly6im9ih9fpycp5zq1895y2jl4kzm-linux-libre-5.10.2
 ...
 linux-libre-5.10.2  76.4MiB

downloading from 
https://ci.guix.gnu.org/nar/lzip/gw4nybiqbfg7mayvykvrk4vd7jf2liyx-linux-libre-module-builder-5.10.2
 ...
 linux-libre-module-builder-5.10.2  17.4MiB
```