drop downstream XFS nommu changes

[ddiss]

With the XFS vm_map_ram() -> vmalloc() conversion from https://lore.kernel.org/linux-xfs/20250310131917.552600-1-hch@lst.de/ now in mainline (v6.15+), we should now be able to revert downstream commit 8ebe11e ("xfs: support for non-mmu architectures").