nfsd: refine and rename NFSD_MAY_LOCK

NFSD_MAY_LOCK means a few different things.
- it means that GSS is not required.
- it means that with NFSEXP_NOAUTHNLM, authentication is not required
- it means that OWNER_OVERRIDE is allowed.

None of these are specific to locking, they are specific to the NLM
protocol.
So:
 - rename to NFSD_MAY_NLM
 - set NFSD_MAY_OWNER_OVERRIDE and NFSD_MAY_BYPASS_GSS in nlm_fopen()
   so that NFSD_MAY_NLM doesn't need to imply these.
 - move the test on NFSEXP_NOAUTHNLM out of nfsd_permission() and
   into fh_verify where other special-case tests on the MAY flags
   happen.  nfsd_permission() can be called from other places than
   fh_verify(), but none of these will have NFSD_MAY_NLM.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>