Use existing swap() function rather than duplicating its implementation.

./lib/crypto/mpi/mpi-pow.c:211:11-12: WARNING opportunity for swap().
./lib/crypto/mpi/mpi-pow.c:239:12-13: WARNING opportunity for swap().
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=9327Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>