NFS: Allow very small rsize & wsize again

940261a1

 introduced nfs_io_size() to clamp the iosize to a multiple
of PAGE_SIZE. This had the unintended side effect of no longer allowing
iosizes less than a page, which could be useful in some situations.

UDP already has an exception that causes it to fall back on the
power-of-two style sizes instead. This patch adds an additional
exception for very small iosizes.

Reported-by: Jeff Layton <jlayton@kernel.org>
Fixes: 940261a1

 ("NFS: Allow setting rsize / wsize to a multiple of PAGE_SIZE")
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>