NFSv4/pNFS: Fix another issue with a list iterator pointing to the head

In nfs4_callback_devicenotify(), if we don't find a matching entry for
the deviceid, we're left with a pointer to 'struct nfs_server' that
actually points to the list of super blocks associated with our struct
nfs_client.
Furthermore, even if we have a valid pointer, nothing pins the super
block, and so the struct nfs_server could end up getting freed while
we're using it.

Since all we want is a pointer to the struct pnfs_layoutdriver_type,
let's skip all the iteration over super blocks, and just use APIs to
find the layout driver directly.

Reported-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Fixes: 1be5683b ("pnfs: CB_NOTIFY_DEVICEID")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>