NFS: Fix O_DIRECT locking issues

[ Upstream commit 7c633932 ]

The dreq fields are protected by the dreq->lock.

Fixes: 954998b6 ("NFS: Fix error handling for O_DIRECT write scheduling")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>