nios2: implement flush_dcache_mmap_lock/unlock

Use spin_lock/unlock_irq instead of doing nothing. This fixes corruptions
of the vma_interval_tree causing the kernel to be stuck in an
infinite loop in vma_interval_tree_foreach.

Signed-off-by: Julien Beraud <julien.beraud@spectracom.orolia.com>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>