xfs: kill XBF_UNMAPPED

Unmapped buffer access is a pain, so kill it. The switch to large
folios means we rarely pay a vmap penalty for large buffers,
so this functionality is largely unnecessary now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>