net: Stop including NFS overhead in defragment max

At least on bfin, this "specimen" is actually allocated in the BSS and
wastes lots of memory in already tight memory conditions.

Also, with the introduction of NFSv3 support, this waste got
substantially larger.

Just remove it. If a board needs a specific different defragment size,
that board can override this setting.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>