nfsd: return errors, not NULL, from export functions

I converted the various export-returning functions to return -ENOENT instead
of NULL, but missed a few cases.

This particular case could cause actual bugs in the case of a krb5 client that
doesn't match any ip-based client and that is trying to access a filesystem
not exported to krb5 clients.

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