lib/rbtree: add random seed

Current test use pseudo rand function with fixed seed, which means the
test data is the same pattern each time.

Add random seed parameter to randomize the test.

Link: https://lkml.kernel.org/r/20250310074938.26756-4-richard.weiyang@gmail.com


Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michel Lespinasse <michel@lespinasse.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>