[$] get_user_pages(), pinned pages, and DAX

The problems associated with the kernel's internal get_user_pages()
function have been a topic of discussion at the Linux Storage, Filesystem,
and Memory-Management Summit for a few years. At the 2019 event,
Jan Kara began a plenary session by saying that it would be 'like last
year's session'. It turned out rather differently, though, perhaps due to
the plenary setting; this discussion (along with the related session that
followed) turned out to be one of the most heated at the entire conference.