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

Commit fbe77c30 ("NFS: move rw_mode to nfs_pageio_header")
reintroduced some pointless code that commit 518662e0 ("NFS: fix
usage of mempools.") had recently removed.

Remove it again.

Cc: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>