knfsd: Allow NFSv2/3 WRITE calls to succeed when krb5i etc is used.

When RPCSEC/GSS and krb5i is used, requests are padded, typically to a multiple
of 8 bytes.  This can make the request look slightly longer than it
really is.

As of

	f34b9568

 "The NFSv2/NFSv3 server does not handle zero
		length WRITE request correctly",

the xdr decode routines for NFSv2 and NFSv3 reject requests that aren't
the right length, so krb5i (for example) WRITE requests can get lost.

This patch relaxes the appropriate test and enhances the related comment.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Peter Staubach <staubach@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>