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
pkg/sysinfo/numcpu_linux.go
package sysinfo // import "github.com/docker/docker/pkg/sysinfo"

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)
	if err != nil {
		return 0
	}

	return mask.Count()
}

package sysinfo // import "github.com/docker/docker/pkg/sysinfo"

import "golang.org/x/sys/unix"

// numCPU queries the system for the count of threads available
// for use to this process.
//
// 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.
	var mask unix.CPUSet
	err := unix.SchedGetaffinity(0, &mask)
	if err != nil {
		return 0
	}
	return mask.Count()
}

This line can be just removed
