nfsd: Cache R, RW, and W opens separately

The nfsd open code has always kept separate read-only, read-write, and
write-only opens as necessary to ensure that when a client closes or
downgrades, we don't retain more access than necessary.

Also, I didn't realize the cache behaved this way when I wrote
94415b06 "nfsd4: a client's own opens needn't prevent delegations".
There I assumed fi_fds[O_WRONLY] and fi_fds[O_RDWR] would always be
distinct.  The violation of that assumption is triggering a
WARN_ON_ONCE() and could also cause the server to give out a delegation
when it shouldn't.

Fixes: 94415b06

 ("nfsd4: a client's own opens needn't prevent delegations")
Tested-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>