Do not allow to use O_BENEATH as an oracle.

Specifically, if lookup() returned any error and the topping directory
was not latched, which means that (non-existent) path did not returned
to the topping location, give ENOTCAPABLE a priority over the lookup()
error.

PR: 249960
Reviewed by: emaste, ngie
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D26695