libceph: use swap() macro instead of taking tmp variable

Fix the following coccicheck warning:
net/ceph/crush/mapper.c:1077:8-9: WARNING opportunity for swap()

by using swap() for the swapping of variable values and drop
the tmp variable that is not needed any more.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>