Looking in that log file, there are a small number of incompatible pointer type
errors:
/usr/src/linux-headers-4.16.0-1-common/include/linux/posix_acl.h:45:16: error: passing argument 1 of ‘refcount_inc’ from incompatible pointer type [-Werror=incompatible-pointer-types]
refcount_inc(&acl->a_refcount);
[...]
/var/lib/dkms/zfs/0.7.6/build/include/linux/vfs_compat.h:288:26: error: passing argument 2 of ‘atomic_sub_return’ from incompatible pointer type [-Werror=incompatible-pointer-types]
if (atomic_dec_and_test(&acl->a_refcount))
and a few others. I don't know if I should try to rejigger it to not error on
that condition, or if this indicates some problem. I'd like to hear from others
that have built linux kernel modules on sparc64 if they have any idea...