gh-96026: do not unregister without register in shm POSIX error handling

`multiprocessing.shared_memory.SharedMemory` currently tries to unlink a POSIX
shared memory object if it fails to mmap it after opening/creating it in the
constructor. It does this by calling `cleanup()`, which unregisters the shared
memory despite it not having been registered. This will trigger a separate
exception in the resource tracker.

Fix this by calling the low-level unlink directly.

Author: duaneg

Lib/multiprocessing/shared_memory.py

@@ -116,7 +116,7 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
                 size = stats.st_size
                 self._mmap = mmap.mmap(self._fd, size)
             except:
-                self.unlink()
+                _posixshmem.shm_unlink(self._name)
                 raise
             if self._track:
                 resource_tracker.register(self._name, "shared_memory")