NFSv4.1: Fix layoutcommit error handling

Firstly, task->tk_status will always return negative error values,
so the current tests for 'NFS4ERR_DELEG_REVOKED' etc. are all being
ignored.
Secondly, clean up the code so that we only need to test
task->tk_status once!

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org