Point dkms build at installed source tree, rather than build directory.

Signed-off-by: Tom Prince <tom.prince@clusterhq.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#2776

scripts/dkms.mkconf

@@ -27,7 +27,7 @@ PRE_BUILD="configure
   --with-config=kernel
   --with-linux=\${kernel_source_dir}
   --with-linux-obj=\${kernel_source_dir}
-  --with-spl=\${dkms_tree}/spl/\${PACKAGE_VERSION}/build
+  --with-spl=\${$source_tree}/spl-\${PACKAGE_VERSION}/
   --with-spl-obj=\${dkms_tree}/spl/\${PACKAGE_VERSION}/\${kernelver}/\${arch}
   --with-spl-timeout=300
   \$(