pkg/sysinfo: omit Getpid call in numCPU

The man page for sched_setaffinity(2) states the following about the pid
argument [1]:

> If pid is zero, then the mask of the calling thread is returned.

Thus the additional call to unix.Getpid can be omitted and pid = 0
passed to unix.SchedGetaffinity.

[1] https://man7.org/linux/man-pages/man2/sched_setaffinity.2.html#DESCRIPTION

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

pkg/sysinfo/numcpu_linux.go

@@ -1,23 +1,17 @@
 package sysinfo // import "github.com/docker/docker/pkg/sysinfo"
 
-import (
-	"golang.org/x/sys/unix"
-)
+import "golang.org/x/sys/unix"
 
 // numCPU queries the system for the count of threads available
 // for use to this process.
 //
-// Issues two syscalls.
 // Returns 0 on errors. Use |runtime.NumCPU| in that case.
 func numCPU() int {
 	// Gets the affinity mask for a process: The very one invoking this function.
-	pid := unix.Getpid()
-
 	var mask unix.CPUSet
-	err := unix.SchedGetaffinity(pid, &mask)
+	err := unix.SchedGetaffinity(0, &mask)
 	if err != nil {
 		return 0
 	}
-
 	return mask.Count()
 }