nfsd: Add sysctl to set maximum I/O size up to 1Mbyte

Since MAXPHYS now allows the FreeBSD NFS client
to do 1Mbyte I/O operations, add a sysctl called vfs.nfsd.srvmaxio
so that the maximum NFS server I/O size can be set up to 1Mbyte.
The Linux NFS client can also do 1Mbyte I/O operations.

The default of 128Kbytes for the maximum I/O size has
not been changed for two reasons:

• kern.ipc.maxsockbuf must be increased to support 1Mbyte I/O
• The limited benchmarking I can do actually shows a drop in I/O rate when the I/O size is above 256Kbytes.

However, daveb@spectralogic.com reports seeing an increase
in I/O rate for the 1Mbyte I/O size vs 128Kbytes using a Linux client.

Reviewed by: asomers
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30826