fuse: use an unrestricted backing device with iomap pagecache io

With iomap support turned on for the pagecache, the kernel issues
writeback to directly to block devices and we no longer have to push all
those pages through the fuse device to userspace.  Therefore, we don't
need the tight dirty limits (~1M) that are used for regular fuse.  This
dramatically increases the performance of fuse's pagecache IO.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>