Fix compile-time warnings caused by duplicate struct typedefs

[kusumi]

Some compiler/versions warn these typedefs according to #16660.

The platform specific header sys/abd_os.h shouldn't define or use abd_t, as it's defined in its non-platform specific consumer sys/abd.h. Do the same as what FreeBSD header does.

Motivation and Context

Fixes #16660 (first one mentioned there)

Description

Fixes #16660

How Has This Been Tested?

Compiled on Linux.

Types of changes

• [x] 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:

• [x] My code follows the OpenZFS code style requirements.
• [ ] I have updated the documentation accordingly.
• [x] 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.
• [x] All commit messages are properly formatted and contain Signed-off-by.