[MIPS] sys_mmap2 offset argument should always be shifted 12, not PAGE_SHIFT.

    
This patch adjusts the offset argument passed into sys_mmap2 to be
always shifted 12, even when the native page size isn't 4K.  This is
what all existing userspace libraries expect.
    
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

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