Use fallthrough macro (2.1 edition)

[rincebrain]

Motivation and Context

Debug building is broken on e.g. Fedora 35 without it. (Waves to #12667)

Description

Just kicking 6954c22 until it applied to 2.1. Nothing fancy, though I was impressed with how badly git cherry-pick got confused by some of them initially.

How Has This Been Tested?

Well, without this, 2.1-release won't build with --enable-debug on F35, and with this, it will.

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.

[tonyhutter]

@rincebrain would you mind making a trivial update to get around the conflict? I think this is all you need for zfs_context_os.h:

diff --git a/include/os/linux/zfs/sys/zfs_context_os.h b/include/os/linux/zfs/sys/zfs_context_os.h
index 981a6b8..9e42605 100644
--- a/include/os/linux/zfs/sys/zfs_context_os.h
+++ b/include/os/linux/zfs/sys/zfs_context_os.h
@@ -25,6 +25,7 @@
 
 #include <linux/dcache_compat.h>
 #include <linux/utsname_compat.h>
+#include <linux/compiler_compat.h>
 #include <linux/module.h>
 
 #if THREAD_SIZE >= 16384

[rincebrain]

Hopefully I didn't accidentally rewrite history in a way that merging the branch will break the world, but there.

edit: I needn't have bothered looking at the CI results; without #12700 and #12673, this wouldn't have passed anyway.