x86: ptrace FLAG_MASK cleanup

This cleans up the FLAG_MASK macro to use symbolic constants instead of a
magic number.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>