UML on UML fixed: it did not start

It is currently impossible to run a user-mode linux machine inside another
user-mode linux (UML on UML).  It breaks after a few instructions.  When
it tries to check whether SYSEMU is installed (the inner) UML receives an
inconsistent result (from the outer UML).

This is the output of a broken attempt:
$ ./linux mem=256m ubd0=cow
Locating the bottom of the address space ... 0x0
Locating the top of the address space ... 0xc0000000
Core dump limits :
        soft - 0
        hard - NONE
Checking that ptrace can change system call numbers...OK
Checking ptrace new tags for syscall emulation...unsupported
Checking syscall emulation patch for ptrace...check_sysemu : expected SIGTRAP, got status = 256
$

The problem is the following:

PTRACE_SYSCALL/SINGLESTEP is currently managed inside arch_ptrace for ARCH=um.

PTRACE_SYSEMU/SUSEMU_SINGLESTEP is not captured in arch_ptrace's switch,
therefore it is erroneously passed back to ptrace_request (in
kernel/ptrace).

This simple patch simply forces ptrace to return an error on
PTRACE_SYSEMU/SUSEMU_SINGLESTEP as it is unsupported on ARCH=um, and fixes
the problem.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Renzo Davoli <renzo@cs.unibo.it>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>