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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Reviewed-by: Stephen Boyd <[e-mail]>
Cc: Jan Kiszka <[e-mail]>
Cc: Kieran Bingham <[e-mail]>
Cc: Aymeric Agon-Rambosson <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Link: https://adamj.eu/tech/2020/01/21/why-does-python-3-8-syntaxwarning-for-is-literal/
Signed-off-by: Linus Torvalds <[e-mail]>

scripts/gdb/linux/rbtree.py

···
         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']:
···
         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']: