client: fix compile warning and incorrect computing the max_fwd

client: fix compile warning and incorrect computing the max_fwd

warning: suggest parentheses around ‘-’ inside ‘<<’ [-Wparentheses]
   max_fwd = 1 << (max_fwd * CHAR_BIT) - 1;
                  ~~~~~~~~~~~~~~~~~~~~~^~~

And also the '-' has precedence over the '<<', more detail please
see https://en.cppreference.com/w/c/language/operator_precedence.

Fixes: https://tracker.ceph.com/issues/55409
Reported-by: Jos Collin <jcollin@redhat.com>
Reported-by: Rishabh Dave <ridave@redhat.com>
Signed-off-by: Xiubo Li <xiubli@redhat.com>