knfsd: eliminate unnecessary -ENOENT returns on export downcalls

A succesful downcall with a negative result (which indicates that the given
filesystem is not exported to the given user) should not return an error.

Currently mountd is depending on stdio to write these downcalls.  With some
versions of libc this appears to cause subsequent writes to attempt to write
all accumulated data (for which writes previously failed) along with any new
data.  This can prevent the kernel from seeing responses to later downcalls.
Symptoms will be that nfsd fails to respond to certain requests.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>