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 7ac592aa35a684ff ("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/[email protected]
Suggested-by: Christian Brauner <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Signed-off-by: Eugene Syromiatnikov <[email protected]>
Complements: 7ac592aa35a684ff ("sched: prctl() core-scheduling interface")
Signed-off-by: Christian Brauner <[email protected]>