librbd: return area from extents_to_file()

librbd: return area from extents_to_file()

Note that, as suggested by extents_to_file() signature, all returned
image extents would pertain to the same area.  This means that an area
boundary must coincide with an object boundary.

luks::FormatRequest is actually more strict: crypto header area size is
set to a multiple of stripe period (i.e. one or more whole objects).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>