Should be merged go into the previous patch.

Also I just noticed how I misread ->mmap_validate in the first patch - it
doesn't just validate the mmap arguments but also replaces ->mmap
itself.

Which means that a) the name for it is very confusing, and b) we should
at very least have it fully replace ->mmap for file systems that
implement it, that is ext4 and xfs should not have to implement both
but just the new one.