MFC: r290970
mnt_stat.f_iosize (which is used to set bo_bsize) must be set to
the largest size of buffer cache block or the mapping of the buffer
is bogus. When a mount with rsize=4096,wsize=4096 was done, f_iosize
would be set to 4096. This resulted in corrupted directory data, since
the buffer cache block size for directories is NFS_DIRBLKSIZ (8192).
This patch fixes the code so that it always sets f_iosize to at least
NFS_DIRBLKSIZ.