inject experimental kernel flag into dkms

[n0xena]

So according #583 (comment) there's no need to add the missing configure parameters to the dkms build.
Hence this is a blueprint for adding the experimental flag to the dkms build - simply because I just don't want to keep dragging along the PR branch.

1. Adding a patch file to the archzfs/src/zfs-dkms/ directory:
   dkms-configure.patch

diff -ura a/scripts/dkms.mkconf b/scripts/dkms.mkconf
--- a/scripts/dkms.mkconf       2025-03-11 20:57:12.078718755 +0100
+++ b/scripts/dkms.mkconf       2025-03-11 20:59:19.611532953 +0100
@@ -26,6 +26,7 @@
 PACKAGE_CONFIG="${pkgcfg}"
 NO_WEAK_MODULES="yes"
 PRE_BUILD="configure
+  --enable-linux-experimental
   --disable-dependency-tracking
   --prefix=/usr
   --with-config=kernel

2. Adding the patch file to archzfs/src/zfs/dkms/PKGBUILD.sh (for now we just skip the hash)

source=("${zfs_src_target}"
    "dkms-configure.patch")
sha256sums=("${zfs_src_hash}"
    SKIP)
// ...
package() {
    dkmsdir="\${pkgdir}/usr/src/zfs-${zfs_mod_ver}"
    install -d "\${dkmsdir}"
    cd ${zfs_workdir}
    patch -Np1 -i ../dkms-configure.patch
    cp -a ${zfs_workdir}/. \${dkmsdir}

    cd "\${dkmsdir}"
    find . -name ".git*" -print0 | xargs -0 rm -fr --
    scripts/dkms.mkconf -v ${zfs_mod_ver} -f dkms.conf -n zfs
}

3. bump pkgrel version in archzfs/src/kernels/dkms.sh

pkgrel="(+1 from current pkgrel)"

In the end currently with 2.3.1 official supporting 6.13 this is pretty much obsolete - for now. But 6.14 is planed for end of march and according to openzfs/zfs#17026 (comment) 6.14 should work with 2.3.1 as well as 6.13 did with 2.3.0.
And as this repos' mantra is to provide something stable this might be non relevant at all.