nfs/localio: eliminate unnecessary kref in nfs_local_fsync_ctx

nfs_local_commit() doesn't need async cleanup of nfs_local_fsync_ctx,
so there is no need to use a kref.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>