x86/kernel: Use kstack_end() in dumpstack_64.c

x86/kernel: Use kstack_end() in dumpstack_64.c

i386 is already using kstack_end() in dumpstack_32.c. We should also
use it to make the code clearer and unify the stack printing logic some
more.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Adrien Schildknecht <adrien+dev@schischi.me>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: c: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1424618638-6375-1-git-send-email-adrien+dev@schischi.me
Signed-off-by: Borislav Petkov <bp@suse.de>