NFSv4: Fix an Oops in pnfs_mark_request_commit() when doing O_DIRECT

Fix an Oopsable condition in pnfs_mark_request_commit() when we're
putting a set of writes on the commit list to reschedule them after a
failed pNFS attempt.

Fixes: 9c455a8c ("NFS/pNFS: Clean up pNFS commit operations")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>