abd_get_offset_struct() may allocate new abd

[lundman]

ABD can be initialised twice - which is a panic on some platforms.

Motivation and Context

Ensure any call to abd_init_struct(); has a corresponding call to abd_fini_struct();

Description

Even when supplied with an abd to abd_get_offset_struct(), the call
to abd_get_offset_impl() can allocate a different abd. Ensure to
call abd_fini_struct() on the abd that is not used.

This was easily triggered on macOS with zpool_upgrade zfs-tests. With the stack:

 : mach_kernel : _panic + 0x54
 : _abd_get_offset_struct.cold.1 + 0x37
 : _abd_get_offset_struct + 0x8b
 : _vdev_raidz_map_alloc + 0x2e1
 : _vdev_raidz_io_start + 0x33
 : _zio_vdev_io_start + 0x21c
 : _zio_nowait + 0x20c

How Has This Been Tested?

Lots of tests on macOS - now to make sure it doesn't break for others.

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.

[behlendorf]

Looks good. Seems like this issue could be a problem for all the platforms to varying degrees.

[ahrens]

Thanks for catching this!