ceph: fix invalid point dereference for error case in mdsc destroy

ceph: fix invalid point dereference for error case in mdsc destroy

1. set fsc->mdsc after successfully allocate all necessary memory
in mdsc init.
2. if fsc->mdsc is NULL, just skip destroy operation in mdsc destroy.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>