Drop reference on shared anon mappable

We call NewSharedAnonMappable simply to use it for Mappable/MappingIdentity for
shared anon mmap. From MMapOpts.MappingIdentity: "If MMapOpts is used to
successfully create a memory mapping, a reference is taken on MappingIdentity."

mm.createVMALocked (below) takes this additional reference, so we don't need
the reference returned by NewSharedAnonMappable. Holding it leaks the mappable.

PiperOrigin-RevId: 241038108
Change-Id: I78ee3af78e0cc7aac4063b274b30d0e41eb5677d

pkg/sentry/mm/syscalls.go

@@ -101,6 +101,7 @@ func (mm *MemoryManager) MMap(ctx context.Context, opts memmap.MMapOpts) (userme
 			if err != nil {
 				return 0, err
 			}
+			defer m.DecRef()
 			opts.MappingIdentity = m
 			opts.Mappable = m
 		}