NFS - Fix BIO size panics

NFS - Fix BIO size panics

* NFS calculates the BIO block size and stores it in mnt_stat.f_iosize.
  This field was getting overwritten with default values by STATFS
  being passing &mp->mnt_stat from the kernel.

  Conditionalize the field so it no longer gets overwritten, and also
  correct nm_rsize/nm_wsize tests to use the smaller of the two
  specifications for the BIO block size.

* Fixes diskless NFS root mounts, and NFS mounts which use smaller
  blocksizes.

Reported-by: Rumko <rumcic@gmail.com>, and others