[PATCH] knfsd: Allow max size of NFSd payload to be configured

The max possible is the maximum RPC payload.  The default depends on amount of
total memory.

The value can be set within reason as long as no nfsd threads are currently
running.  The value can also be ready, allowing the default to be determined
after nfsd has started.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>