NFS: ncommit count is being double decremented

The decrement is handled by each call to nfs_request_remove_commit_list,
no need to do it again in nfs_scan_commit.

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