MIPS: cache: Provide cache flush operations for XFS
Until now flush_kernel_vmap_range() and invalidate_kernel_vmap_range() did
not exist on MIPS resulting in heavy cache corruption on XFS filesystems.
Left for the post-3.0 time: optimization and make this work with highmem,
too. Since the combination of highmem + cache aliases atm doesn't work
this isn't a regression.