uapi/linux/prctl: provide macro definitions for the PR_SCHED_CORE type argument

uapi/linux/prctl: provide macro definitions for the PR_SCHED_CORE type argument

Commit 48b54077be6b1a0d ("sched: prctl() core-scheduling interface")
made use of enum pid_type in prctl's arg4; this type and the associated
enumeration definitions are not exposed to userspace.  Christian
has suggested to provide additional macro definitions that convey
the meaning of the type argument more in alignment with its actual
usage, and this patch does exactly that.

Link: https://lore.kernel.org/r/20210825170613.GA3884@asgard.redhat.com
Suggested-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Complements: 48b54077be6b1a0d ("sched: prctl() core-scheduling interface")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>