NFSv4.1/flexfiles: Fix freeing of mirrors

Mirrors are now shared objects, so we should not be freeing them directly
inside ff_layout_free_lseg(). We should already be doing the right thing
in _ff_layout_free_lseg(), so just let it handle things.

Also ensure that ff_layout_free_mirror() frees the RPC credential if it
is set.

Fixes: 28a0d72c ("Add refcounting to struct nfs4_ff_layout_mirror")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>