knfsd: avoid use of unitialised variables on error path when nfs exports

We need to zero various parts of 'exp' before any 'goto out', otherwise when
we go to free the contents...  we die.

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