[MIPS] Fix BUG(), BUG_ON() handling

With commit 63dc68a8

, kernel can not
handle BUG() and BUG_ON() properly since get_user() returns false for
kernel code.  Use __get_user() to skip unnecessary access_ok().  This
patch also make BRK_BUG code encoded in the TNE instruction.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>