x86/mm/pat: Use rb_entry()

To make the code clearer, use rb_entry() instead of open coding it

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Link: http://lkml.kernel.org/r/974a91cd4ed2d04c92e4faa4765077e38f248d6b.1482157956.git.geliangtang@gmail.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>