graid3: Fix teardown in g_raid3_try_destroy()

Commit 33cb9b3c3a22 replaced a g_raid3_destroy_device() call with a
g_raid3_free_device() call, which was incorrect and could lead to a
panic if a RAID3 GEOM failed to start (e.g., due to missing disks).

Reported by: graid3 tests
Fixes: 33cb9b3c3a22 ("graid3: Fix teardown races")
MFC after: 3 days
Sponsored by: Klara, Inc.