fs/cifs/cifs_dfs_ref.c: fix potential memory leakage

When it goes to error through line 144, the memory allocated to *devname is
not freed, and the caller doesn't free it either in line 250. So we free the
memroy of *devname in function cifs_compose_mount_options() when it goes to
error.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
CC: stable <stable@kernel.org>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>