NFS: Make nfs_commitdata_release static

The symbol nfs_commitdata_release is only used locally
in this file. Make it static to prevent the following sparse warning:

warning: symbol 'nfs_commitdata_release' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>