NFSv4.1: Clean up the removal of pnfs_layout_hdr from the server list

Move the code into pnfs_free_layout_hdr(), and add checks to
get_layout_by_fh_locked to ensure that they don't reference a layout
that is being freed.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>