mm/nommu.c: drop unlikely inside BUG_ON()

mm/nommu.c: drop unlikely inside BUG_ON()

(1) For !CONFIG_BUG cases, the bug call is a no-op, so we couldn't
    care less and the change is ok.

(2) ppc and mips, which HAVE_ARCH_BUG_ON, do not rely on branch
    predictions as it seems to be pointless[1] and thus callers should not
    be trying to push an optimization in the first place.

(3) For CONFIG_BUG and !HAVE_ARCH_BUG_ON cases, BUG_ON() contains an
    unlikely compiler flag already.

Hence, we can drop unlikely behind BUG_ON().

[1] http://lkml.iu.edu/hypermail/linux/kernel/1101.3/02289.html

Signed-off-by: Geliang Tang <geliangtang@163.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>