ceph: fix inode reference leakage in ceph_get_snapdir()

The ceph_get_inode() will search for or insert a new inode into the
hash for the given vino, and return a reference to it. If new is
non-NULL, its reference is consumed.

We should release the reference when in error handing cases.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>