staging: android: ashmem: Fix a race condition in pin ioctls

commit ce8a3a9e76d0193e2e8d74a06d275b3c324ca652 upstream.

ashmem_pin_unpin() reads asma->file and asma->size before taking the
ashmem_mutex, so it can race with other operations that modify them.

Build-tested only.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>