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