NFS: Use the correct commit info in nfs_join_page_group()

Ensure that nfs_clear_request_commit() updates the correct counters when
it removes them from the commit list.

Fixes: ed5d588f

 ("NFS: Try to join page groups before an O_DIRECT retransmission")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>