NFSv4/pnfs: Fix an infinite layoutget loop

Since we can now use a lock stateid or a delegation stateid, that
differs from the context stateid, we need to change the test in
nfs4_layoutget_handle_exception() to take this into account.

This fixes an infinite layoutget loop in the NFS client whereby
it keeps retrying the initial layoutget using the same broken
stateid.

Fixes: 70d2f7b1

 ("pNFS: Use the standard I/O stateid when...")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>