staging: android: ashmem: Disallow ashmem memory from being remapped

When ashmem file is being mmapped the resulting vma->vm_file points to the
backing shmem file with the generic fops that do not check ashmem
permissions like fops of ashmem do. Fix that by disallowing mapping
operation for backing shmem file.

Bug: 142903466
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>