Backport of #13236 for zfs-2.1.4 staging branch

[behlendorf]

Motivation and Context

Issue #13210

Description

Commit 3b52ccd introduced a flaw where FSR and FSAVE are not restored
when using a Linux 5.16 kernel. These instructions are only used when
XSAVE is not supported by the processor meaning only some systems will
encounter this issue.

How Has This Been Tested?

This change needs to be verified on hardware which does not support XSAVE.

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.

[AttilaFueloep]

The same bug is also in master, should I open a PR or will you do so?

[behlendorf]

I'm on it, I've opened #13236 against master and renamed this as the backport.