Re: [PATCH 6/7] mm, fs: introduce file_operations->post_mmap()

When mappings are created the vma->vm_flags that they use vary based on whether the inode being mapped is using DAX or not. This setup happens in XFS via mmap_region()=>call_mmap()=>xfs_file_mmap(). For us to be able to safely use the DAX per-inode flag we need to prevent S_DAX transitions when any mappings are present, and we will do that by looking at the address_space->i_mmap tree and returning -EBUSY if any mappings are present. Unfortunately at the time that the filesystem's file_operations->mmap() entry point is called the mapping has not yet been added to the address_space->i_mmap tree. This means that at that point in time we cannot determine whether or not the mapping will be set up to support DAX. Fix this by adding a new file_operations entry called post_mmap() which is called after the mapping has been added to the address_space->i_mmap tree. This post_mmap() op now happens at a time when we can be sure whether the mapping will use DAX or not, and we can set up the vma->vm_flags appropriately.