x86/asm/entry: Simplify task_pt_regs() macro definition

Before this change, task_pt_regs() was using KSTK_TOP(),
and it was the only use of that macro. In turn, KSTK_TOP used
THREAD_SIZE_LONGS, and it was the only use of that macro too.

Fold these macros into task_pt_regs(). Tweak comment
about "- 8" - we now use a symbolic constant, not literal 8.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1426255743-5394-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>