nfs: Fix wrong slab cache in nfs_commit_mempool

The slab cache in nfs_commit_mempool is wrong, and I think it is just a slip.
I tested it on a x86-32 machine, the size of nfs_write_header is 544, and
the size of nfs_commit_data is 408, so it works fine. It is also true that
sizeof(struct nfs_write_header) > sizeof(struct nfs_commit_data) on other
platforms in my opinoin. Just fix it.
Signed-off-by: NYanchuan Nian <ycnian@gmail.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/write.c

@@ -1829,7 +1829,7 @@ int __init nfs_init_writepagecache(void)
 		goto out_destroy_write_mempool;
 
 	nfs_commit_mempool = mempool_create_slab_pool(MIN_POOL_COMMIT,
-						      nfs_wdata_cachep);
+						      nfs_cdata_cachep);
 	if (nfs_commit_mempool == NULL)
 		goto out_destroy_commit_cache;