NFSv4.1/flexfiles: Allow coalescing of new layout segments and existing ones

In order to ensure atomicity of updates, we merge the old layout segments
into the new ones, and then invalidate the old ones.

Also ensure that we order the list of layout segments so that
RO segments are preferred over RW.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>