ceph: implement -o test_dummy_encryption mount option

Add support for the test_dummy_encryption mount option. This allows us
to test the encrypted codepaths in ceph without having to manually set
keys, etc.

[ lhenriques: fix potential fsc->fsc_dummy_enc_policy memory leak in
  ceph_real_mount() ]

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>