crypto: testmgr - fix initialization of 'secret_size'

mainline inclusion
from mainline-master
commit 2d016672
category: bugfix
bugzilla: 173981
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2d016672528a592ada5188e53ac746e1b8b7a978



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Actual data length of the 'secret' is not equal to the 'secret_size'.

Since the 'curve_id' has removed in the 'secret', the 'secret_size'
should subtract the length of the 'curve_id'.

Fixes: 6763f5ea ("crypto: ecdh - move curve_id of ECDH from ...")
Signed-off-by: Hui Tang <tanghui20@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Mingqiang Ling <lingmingqiang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>