Workaround for Linux PowerPC GPL-only cpu_has_feature()

[Low-power]

Motivation and Context

Workaround #14545

Description

Copied from commit message:

Linux since 4.7 makes interface 'cpu_has_feature' to use jump labels on
powerpc if CONFIG_JUMP_LABEL_FEATURE_CHECKS is enabled, in this case however
this inline function references GPL-only symbol 'cpu_feature_keys'.

ZFS currently uses 'cpu_has_feature' either directly or indirectly from
several places; while it is unknown how this issue didn't break ZFS on 64-bit
little-endian powerpc, it is known to break ZFS with many Linux versions on
both 32-bit and 64-bit big-endian powerpc.

Until this issue is fixed in Linux, we have to workaround it by providing our
own version of the inline function without using jump labels.

How Has This Been Tested?

It has been tested with a root-on-ZFS configuration using linux-image-6.1.0-6-powerpc64 from Debian, as well as another system using Linux 4.1.42 to ensure it didn't break older Linux versions that didn't affected by the bug.

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.

[Low-power]

I just borrowed your comment and created a #define for cpu_has_feature in the only place it should be needed.

I actually plan to do the same originally, but unfortunately it didn't work, due to the reference by flush_dcache_page. This is an inline function defined in arch/powerpc/include/asm/cacheflush.h of Linux source tree, a later #define can't override cpu_has_feature referenced already by it.

[behlendorf]

I see, it was exported as a symbol until the 5.13 kernel where it was converted to a static inline. That's why I missed it. In that case, how about we #define our own version of flush_dcache_page when flush_dcache_page isn't available to us. It's a simple enough function and hasn't changed since it was moved to the header in 5.13. Then since we're defining it it will be able to use our cpu_has_feature macro wrapper.

[behlendorf]

Looks good, thanks for sorting this out. Just one comment inline.