fuse: handle U64_MAX correctly in DLM locked range index

When merging and checking overlap U64_MAX was not handled
this will create problems on multiple DLM results that cover
the whole range.

Add a fast path for adding and checking locked ranges when
the whole range is covered.

Refactor dlm range handling to make it more readable.

Signed-off-by: Horst Birthelmer <hbirthelmer@ddn.com>

Author: hbirth