NFS/flexfiles: refactor calls to fs4_ff_layout_prepare_ds()

While we may want to skip attempting to connect to a downed mirror
when we're deciding which mirror to select for a read, we do not
want to do so once we've committed to attempting the I/O in
ff_layout_read/write_pagelist(), or ff_layout_initiate_commit()

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