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 449ca0c9 ("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/linux/rbtree.py

@@ -17,7 +17,7 @@ def rb_first(root):
         raise gdb.GdbError("Must be struct rb_root not {}".format(root.type))
 
     node = root['rb_node']
-    if node is 0:
+    if node == 0:
         return None
 
     while node['rb_left']:
@@ -33,7 +33,7 @@ def rb_last(root):
         raise gdb.GdbError("Must be struct rb_root not {}".format(root.type))
 
     node = root['rb_node']
-    if node is 0:
+    if node == 0:
         return None
 
     while node['rb_right']: