um: Fix PTRACE_POKEUSER on x86_64

This is broken since ever but sadly nobody noticed.
Recent versions of GDB set DR_CONTROL unconditionally and
UML dies due to a heap corruption. It turns out that
the PTRACE_POKEUSER was copy&pasted from i386 and assumes
that addresses are 4 bytes long.

Fix that by using 8 as address size in the calculation.

Cc: <[e-mail]>
Reported-by: jie cao <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/x86/um/ptrace_64.c

···
 	else if ((addr >= offsetof(struct user, u_debugreg[0])) &&
 		(addr <= offsetof(struct user, u_debugreg[7]))) {
 		addr -= offsetof(struct user, u_debugreg[0]);
-		addr = addr >> 2;
+		addr = addr >> 3;
 		if ((addr == 4) || (addr == 5))
 			return -EIO;
 		child->thread.arch.debugregs[addr] = data;