fuse: allow servers to use iomap for better file IO performance [RFC[RAP] V2 4/6]

This series connects fuse (the userspace filesystem layer) to fs-iomap to get
fuse servers out of the business of handling file I/O themselves.  By keeping
the IO path mostly within the kernel, we can dramatically improve the speed of
disk-based filesystems.  This enables us to move all the filesystem metadata
parsing code out of the kernel and into userspace, which means that we can
containerize them for security without losing a lot of performance.

This has been running on the djcloud for months with no problems.  Enjoy!

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

fuse: advertise support for iomap

Advertise our new IO paths programmatically by creating an ioctl that
can return the capabilities of the kernel.

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