NFSv4: Fix an Oops in nfs4_do_setattr

NFSv4: Fix an Oops in nfs4_do_setattr

If the user specifies an open mode of 3, then we don't have a NFSv4 state
attached to the context, and so we Oops when we try to dereference it.

Reported-by: Olga Kornievskaia <aglo@umich.edu>
Fixes: 5ca66503523f6 ("NFSv4: change nfs4_do_setattr to take...")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org # v4.10: f5da94fcee152: NFSv4: Only pass the...
Cc: stable@vger.kernel.org # v4.10+