nfs: properly protect nfs_direct_req fields

We protect accesses to the nfs_direct_req fields with the dreq->lock
ever where except nfs_direct_commit_complete.  This isn't a huge deal,
but it does lead to confusion, and we could potentially end up setting
NFS_ODIRECT_RESCHED_WRITES in one thread where we've had an error in
another.  Clean this up to properly protect ->error and ->flags in the
commit completion path.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>