fuse: reserve values for mapping protocol

SETUPMAPPING is a command for use with 'virtiofsd', a fuse-over-virtio
implementation; it may find use in other fuse impelementations as well in
which the kernel does not have access to the address space of the daemon
directly.

A SETUPMAPPING operation causes a section of a file to be mapped into a
memory window visible to the kernel.  The offsets in the file and the
window are defined by the kernel performing the operation.

The daemon may reject the request, for reasons including permissions and
limited resources.

When a request perfectly overlaps a previous mapping, the previous mapping
is replaced.  When a mapping partially overlaps a previous mapping, the
previous mapping is split into one or two smaller mappings.

REMOVEMAPPING is the complement to SETUPMAPPING; it unmaps a range of
mapped files from the window visible to the kernel.

The map_alignment field communicates the alignment constraint for
FUSE_SETUPMAPPING/FUSE_REMOVEMAPPING and allows the daemon to constrain the
addresses and file offsets chosen by the kernel.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>