MIPS: Cleanup arch_get_unmapped_area

As noticed by Kevin Cernekee <cernekee@gmail.com> in
http://www.linux-mips.org/cgi-bin/extract-mesg.cgi?a=linux-mips&m=2011-05&i=BANLkTikq04wuK%3Dbz%2BLieavmm3oDtoYWKxg%40mail.gmail.com

Patchwork: https://patchwork.linux-mips.org/patch/2387/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>