xfrm: Remove redundant fields and related parentheses

The variable err is not necessary in such places. It should be revmoved
for the simplicity of the code. This will cause the double parentheses
to be redundant, and the inner parentheses should be deleted.

The clang_analyzer complains as follows:

net/xfrm/xfrm_input.c:533: warning:
net/xfrm/xfrm_input.c:563: warning:

Although the value stored to 'err' is used in the enclosing expression,
the value is never actually read from 'err'.

Changes in v2:

Modify the title, because v2 removes the brackets.
Remove extra parentheses.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>