Proper support for DESTDIR and INSTALL_MOD_PATH

[jlsalvador]

Motivation and Context

The environment variables DESTDIR and INSTALL_MOD_PATH must
be mutually exclusive.

https://www.gnu.org/prep/standards/html_node/DESTDIR.html
https://www.kernel.org/doc/Documentation/kbuild/modules.txt

Description

This issue was discussed in this Buildroot thread:
https://lists.buildroot.org/pipermail/buildroot/2021-August/621350.html

I saw this behavior in other different projects, as:

• Yocto Project:
  https://www.yoctoproject.org/pipermail/meta-freescale/2013-August/004307.html

• Google IA Coral:
  https://coral.googlesource.com/linux-imx-debian/+/refs/heads/master/debian/rules

For the above reasons, INSTALL_MOD_PATH will be set as DESTDIR
by default.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

Looks good, thanks for fixing this and proving the helpful references.

[edacval]

This will break Arch Linux and derivatives PKGBUILD`s because Arch places kernel modules in /usr/lib/modules : https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=zfs-linux#n53 . Maybe @eli-schwartz or @minextu can suggest an workaround ?

[jlsalvador]

This will break Arch Linux and derivatives PKGBUILD`s because Arch places kernel modules in /usr/lib/modules : https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=zfs-linux#n53 . Maybe @eli-schwartz or @minextu can suggest an workaround ?

29,30c29,32
< source=("https://github.com/zfsonlinux/zfs/releases/download/zfs-${_zfsver}/zfs-${_zfsver}.tar.gz")
< sha256sums=("bd4f48d009f3b5e291390bde62b0131b8bf3fab09f4fc0fa3591b1f2e7074cff")
---
> source=("https://github.com/openzfs/zfs/releases/download/zfs-${_zfsver}/zfs-${_zfsver}.tar.gz"
>       "https://github.com/openzfs/zfs/commit/c9dabebc8eef129ab72bef6e78dbfc06f16c4c4e.patch")
> sha256sums=("bd4f48d009f3b5e291390bde62b0131b8bf3fab09f4fc0fa3591b1f2e7074cff"
>       "SKIP")
33a36,40
> prepare() {
>     cd "${srcdir}/zfs-${_zfsver}"
>     patch --forward --strip=1 --input="${srcdir}/c9dabebc8eef129ab72bef6e78dbfc06f16c4c4e.patch"
> }
> 
53c60
<     make DESTDIR="${pkgdir}" INSTALL_MOD_PATH=/usr install
---
>     make DESTDIR="${pkgdir}" INSTALL_MOD_PATH="${pkgdir}/usr" install

I'm going to publish a PR to https://github.com/archzfs/archzfs
Thanks @edacval for the info!