stop_machine: fill fn_result only in case of error

stop_machine: fill fn_result only in case of error

When stop_machine_run() is called with NR_CPUS as last argument,
fn_result member must be filled only if an error happens since it is
shared across all cpus.

Assume CPU1 detects an error and set fn_result to -1, then CPU2 doesn't
detect an error and set fn_result to 0. The error detected by CPU1 will
be ignored.

Note that in case multiple failures occur on different CPUs, only the
last error will be reported.

Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>