FreeBSD: Fix a lock leak in zvol_geom_open()

[markjdb]

This fixes a straightforward locking bug in zvol_geom_open().

Motivation and Context

I hit a panic while running ZFS tests on arm64, caused by zvol_geom_open() trying to unlock zv->zv_suspend_lock even though zv was NULL.

Description

There is some tricky deadlock avoidance code in this function, it failed to update a flag after dropping the suspend lock. Apply the minimal change required to fix that bug.

How Has This Been Tested?

Re-run the test suite on arm64.

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.
• [ ] 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.