fuse: add support for striped backing

Add the FUSE_BACKING_MAP_CYCLIC flag for FUSE_NOTIFY_BACKING_MAP
notifications to enable creating striped maps.

This is just a set of regular extents that repeats after the end of the
last extent using a striping pattern (i.e. on the target backings the
stripes have a contiguous footprint).

Two additional limits apply compared to non-cyclic extent map population:

 - the size of the extents (chunk size) must be equal

 - the extents must start from zero offset and follow each other without
   gaps.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>