lib/rbtree.c: fix typo in comment of __rb_insert()

lib/rbtree.c: fix typo in comment of __rb_insert()

In case 1, it passes down the BLACK color from G to p and u, and maintains
the color of n.  By doing so, it maintains the black height of the
sub-tree.

While in the comment, it marks the color of n to BLACK.  This is a typo
and not consistents with the code.

This patch fixs this typo in comment.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Acked-by: Michel Lespinasse <walken@google.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>