scripts/gdb: fix python 3.8 SyntaxWarning

Fixes the observed warnings:
scripts/gdb/linux/rbtree.py:20: SyntaxWarning: "is" with a literal. Did
you mean "=="?
if node is 0:
scripts/gdb/linux/rbtree.py:36: SyntaxWarning: "is" with a literal. Did
you mean "=="?
if node is 0:

It looks like this is a new warning added in Python 3.8. I've only seen
this once after adding the add-auto-load-safe-path rule to my ~/.gdbinit
for a new tree.

Fixes: commit 449ca0c95ea2 ("scripts/gdb: add rb tree iterating utilities")
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Aymeric Agon-Rambosson <aymeric.agon@yandex.com>
Link: http://lkml.kernel.org/r/20200805225015.2847624-1-ndesaulniers@google.com
Link: https://adamj.eu/tech/2020/01/21/why-does-python-3-8-syntaxwarning-for-is-literal/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/gdb: repair rb_first() and rb_last()

The current implementations of the rb_first() and rb_last() gdb
functions have a variable that references itself in its instanciation,
which causes the function to throw an error if a specific condition on
the argument is met. The original author rather intended to reference
the argument and made a typo. Referring the argument instead makes the
function work as intended.

Signed-off-by: Aymeric Agon-Rambosson <aymeric.agon@yandex.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Nikolay Borisov <n.borisov.lkml@gmail.com>
Cc: Jackie Liu <liuyun01@kylinos.cn>
Cc: Jason Wessel <jason.wessel@windriver.com>
Link: http://lkml.kernel.org/r/20200427051029.354840-1-aymeric.agon@yandex.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/gdb: add rb tree iterating utilities

Implement gdb functions for rb_first(), rb_last(), rb_next(), and
rb_prev(). These can be useful to iterate through the kernel's
red-black trees.

[swboyd@chromium.org: v2]
Link: http://lkml.kernel.org/r/20190329220844.38234-4-swboyd@chromium.org
Link: http://lkml.kernel.org/r/20190325184522.260535-4-swboyd@chromium.org
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Nikolay Borisov <n.borisov.lkml@gmail.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jackie Liu <liuyun01@kylinos.cn>
Cc: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>