NFS: Don't clear desc->pg_moreio in nfs_do_recoalesce()

Recoalescing does not affect whether or not we've already sent off
I/O, and doing so means that we end up sending a bunch of synchronous
for cases where we actually need to be using unstable writes.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>