ZFS: fix adding vdevs to very large pools

r323791 changed the return value of zpool_read_label. Error paths that
previously returned 0 began to return -1 instead. However, not all error
paths initialized errno. When adding vdevs to a very large pool, errno could
be prepopulated with ENOMEM, causing the operation to fail. Fix the bug by
setting errno=ENOENT in the case that no ZFS label is found.

PR: 226096
Submitted by: Nikita Kozlov
Reviewed by: avg
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D13088