ceph_d_revalidate(): propagate stable name down into request encoding

ceph_d_revalidate(): propagate stable name down into request encoding

Currently get_fscrypt_altname() requires ->r_dentry->d_name to be stable
and it gets that in almost all cases.  The only exception is ->d_revalidate(),
where we have a stable name, but it's passed separately - dentry->d_name
is not stable there.

Propagate it down to get_fscrypt_altname() as a new field of struct
ceph_mds_request - ->r_dname, to be used instead ->r_dentry->d_name
when non-NULL.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>