[PATCH] knfsd: fix return value for writes to some files in 'nfsd' filesystem

Most files in the 'nfsd' filesystem are transactional.  When you write, a
reply is generated that can be read back only on the same 'file'.

If the reply has zero length, the 'write' will incorrectly return a value of
'0' instead of the length that was written.  This causes 'rpc.nfsd' to give an
annoying warning.

This patch fixes the test.

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