NFSv4.1/flexfile: Ensure uniqueness of mirrors across layout segments

Keep the full list of mirrors in the struct nfs4_ff_layout_mirror so that
they can be shared among the layout segments that use them.
Also ensure that we send out only one copy of the layoutstats per mirror.

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