uprobes/x86: Introduce sizeof_long(), cleanup adjust_ret_addr() and arch_uretprobe_hijack_return_addr()

1. Add the trivial sizeof_long() helper and change other callers of
   is_ia32_task() to use it.

   TODO: is_ia32_task() is not what we actually want, TS_COMPAT does
   not necessarily mean 32bit. Fortunately syscall-like insns can't be
   probed so it actually works, but it would be better to rename and
   use is_ia32_frame().

2. As Jim pointed out "ncopied" in arch_uretprobe_hijack_return_addr()
   and adjust_ret_addr() should be named "nleft". And in fact only the
   last copy_to_user() in arch_uretprobe_hijack_return_addr() actually
   needs to inspect the non-zero error code.

TODO: adjust_ret_addr() should die. We can always calculate the value
we need to write into *regs->sp, just UPROBE_FIX_CALL should record
insn->length.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jim Keniston <jkenisto@us.ibm.com>