rgw_rest_user.cc: UserQuotas init vars in ctor init list

rgw_rest_user.cc: UserQuotas init vars in ctor init list

Fix for:

[src/rgw/rgw_rest_user.cc:631]: (performance) Variable 'bucket_quota' is
 assigned in constructor body. Consider performing initialization in
 initialization list.
[src/rgw/rgw_rest_user.cc:632]: (performance) Variable 'user_quota' is
 assigned in constructor body. Consider performing initialization in
 initialization list.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>