config/kernel: enforce maximum kernel version, with escape hatch

META lists the maximum kernel version we consider to be fully supported.
However, we don't enforce this.

Sometimes we ship experimental patches for a newer kernel than we're
ready to support or, less often, we compile just fine against a newer
kernel. Invariably, something doesn't quite work properly, and it's
difficult for users to understand that they're actually running against
a kernel that we're not yet ready to support.

This commit tries to improve this situation. First, it simply enforces
Linux-Maximum, by having configure bail out if you try to compile
against a newer version that.

Then, it adds the --enable-linux-experimental switch to configure. When
supplied, this disables enforcing the maximum version, allowing the user
to attempt to build against a kernel with version higher than
Linux-Maximum.

Finally, if the switch is supplied _and_ configure is run against a
higher kernel version, it shows a big warning message when configure
finishes, and defines HAVE_LINUX_EXPERIMENTAL for the build. This allows
us to add code to modify runtime behaviour as well.

Reviewed-by: Robert Evans <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #15986

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