Use new status code from fsadm check

Use new status code from fsadm check

Patch updates exec_cmd() and adds 3rd parameter with pointer for
status value, so caller might examine returned status code.
If the passed pointer is NULL, behavior is unmodified.

Patch allows to confinue with lvresize if the failure from fsadm check is
caused by mounted filesystem as many of filesystem resize tools do support
online filesystem resize. (originally user had to use flag '-n' to bypass
this filesystem check)