zap: reuse zap_leaf_t on dbuf reuse after shrink

[robn]

Motivation and Context

Closes #16157.

Description

If a shrink or truncate had recently freed a portion of the ZAP, the dbuf could still be sitting on the dbuf cache waiting for eviction. If it is then allocated for a new leaf before it can be evicted, the zap_leaf_t is still attached as userdata, tripping the VERIFY.

Instead, just check for the userdata, and if we find it, reuse it.

How Has This Been Tested?

zap_shrink and vdev_zaps (the only two "very obviously ZAPs" test tags) both passed locally.

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.