Support DAX direct access to memory-like storage devices

[tv42]

https://www.kernel.org/doc/html/latest/filesystems/dax.html

FUSE_SETUPMAPPING and FUSE_REMOVEMAPPING.

• https://lwn.net/Articles/877014/
• https://www.phoronix.com/news/Linux-5.10-FUSE-Improvements
• https://www.phoronix.com/news/Linux-5.17-FUSE-Per-inode-DAX

This is going to be low priority unless someone has a specific use case.