ptrace/x86: dont delay "disable" till second pass in ptrace_write_dr7()

ptrace_write_dr7() skips ptrace_modify_breakpoint(disabled => true)
unless second_pass, this buys nothing but complicates the code and means
that we always do the main loop twice even if "disabled" was never true.

The comment says:

	Don't unregister the breakpoints right-away,
	unless all register_user_hw_breakpoint()
	requests have succeeded.

Firstly, we do not do register_user_hw_breakpoint(), it was removed by
commit 24f1e32c ("hw-breakpoints: Rewrite the hw-breakpoints layer
on top of perf events").

We are going to restore register_user_hw_breakpoint() (see the next
patch) but this doesn't matter: after commit 44234adc


("hw-breakpoints: Modify breakpoints without unregistering them")
perf_event_disable() can not hurt, hw_breakpoint_del() does not free the
slot.

Remove the "second_pass" check from the main loop and simplify the code.
Since we have to check "bp != NULL" anyway, the patch also removes the
same check in ptrace_modify_breakpoint() and moves the comment into
ptrace_write_dr7().

With this patch the second pass is only needed to restore the saved
old_dr7.  This should never fail, so the patch adds WARN_ON() to catch
the potential problems as Frederic suggested.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Michael Neuling <mikey@neuling.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>