smack: lsm: remove the unneeded result variable

Return the value smk_ptrace_rule_check() directly instead of storing it
in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Xu Panda <xu.panda@zte.com.cn>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>