[MIPS] Fix non-linear memory mapping on MIPS

    
Fix the non-linear memory mapping done via remap_file_pages() -- it
didn't work on any MIPS CPU because the page offset clashing with
_PAGE_FILE and some other page protection bits which should have been left
zeros for this kind of pages.
    
Signed-off-by: Konstantin Baydarov <kbaidarov@ru.mvista.com>
Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>