CLEANUP: src/cpuset.c: fix missing return in functions returning int

CLEANUP: src/cpuset.c: fix missing return in functions returning int

Cppcheck found the issue described in github #2124, which can cause these
errors if no CPUSET implementation is supported (and CPUSET_USE_ULONG is
not enabled):

src/cpuset.c:21:11: error: Found an exit path from function with non-void return type that has missing return statement [missingReturn]
src/cpuset.c:36:11: error: Found an exit path from function with non-void return type that has missing return statement [missingReturn]
src/cpuset.c:100:1: error: Found an exit path from function with non-void return type that has missing return statement [missingReturn]
src/cpuset.c:124:1: error: Found an exit path from function with non-void return type that has missing return statement [missingReturn]
src/cpuset.c:152:1: error: Found an exit path from function with non-void return type that has missing return statement [missingReturn]
src/cpuset.c:163:1: error: Found an exit path from function with non-void return type that has missing return statement [missingReturn]

This can be backported.