dax: Convert dax_insert_pfn_mkwrite to XArray

Add some XArray-based helper functions to replace the radix tree based
metaphors currently in use.  The biggest change is that converted code
doesn't see its own lock bit; get_unlocked_entry() always returns an
entry with the lock bit clear.  So we don't have to mess around loading
the current entry and clearing the lock bit; we can just store the
unlocked entry that we already have.

Signed-off-by: Matthew Wilcox <willy@infradead.org>