Ignore the cookie verifier for NFSv4.1 when the cookie is 0.

RFC5661 states that the cookie verifier should be 0 when the cookie is 0.
However, the wording is somewhat unclear and a recent discussion on the
nfsv4@ietf.org mailing list indicated that the NFSv4 server should ignore
the cookie verifier's value when the dirctory offset cookie is 0.
This patch deletes the check for this that would return NFSERR_BAD_COOKIE
when the verifier was not 0.
This was found during testing of the ESXi client against the NFSv4.1 server.

Reported by: daniel@ftml.net (via packet trace)
MFC after: 2 weeks