linux/atomic: fill out API for atomic pointer ops

[robn]

[Sponsors: Klara, Inc., Wasabi Technology, Inc.]

Motivation and Context

We have them for FreeBSD and userspace, and I reached for them, and didn't get them on Linux.

Description

Follow the existing pattern for atomic_cas_ptr to produce atomic_swap_ptr, atomic_load_ptr and atomic_save_ptr.

Worth noting that Linux doesn't have standard atomics for pointers, so we can't just wrap those, but they wouldn't be especially different to these anyway.

Asides: probably these can be global, not OS-specific, and possibly they can get some style updates (mainly inline vs __inline__), and it's probably worth thinking about 32-bit support in general. But not today!

How Has This Been Tested?

Compile checked for master. Used on a private project to replace use of atomic_*_64 with some messy casting, and worked just fine. The 32-bit ones, not tested at all.

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.