NFS: fix directio refcount bug on commit

This reverts a hunk from commit 04277086
"NFS: Clean up - Simplify reference counting in fs/nfs/direct.c"

The cleanups in that patch affect the write path, but by the time
processing hits commit the removed reference has been added back by
nfs_scan_commit_list().  Without this reversion, any page that is
sent to commit holds on to an unbalanced reference that is never
freed.  The immediate effect is an imbalance over the wire between
OPENs and CLOSEs.

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>