NFS: don't expect errors from mempool_alloc().

Commit fbe77c30e9ab ("NFS: move rw_mode to nfs_pageio_header")
reintroduced some pointless code that commit 518662e0fcb9 ("NFS: fix
usage of mempools.") had recently removed.

Remove it again.

Cc: Benjamin Coddington <[e-mail]>
Signed-off-by: NeilBrown <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/write.c

···
 {
 	struct nfs_pgio_header *p = mempool_alloc(nfs_wdata_mempool, GFP_NOIO);
 
-	if (p) {
-		memset(p, 0, sizeof(*p));
-		p->rw_mode = FMODE_WRITE;
-	}
+	memset(p, 0, sizeof(*p));
+	p->rw_mode = FMODE_WRITE;
 	return p;
 }