nfsd: nfs4_file_get_access: need to be more careful with O_RDWR

If fi_fds = {non-NULL, NULL, non-NULL} and oflag = O_WRONLY
the WARN_ON_ONCE(!(fp->fi_fds[oflag] || fp->fi_fds[O_RDWR]))
doesn't trigger when it should.

Signed-off-by: Harshula Jayasuriya <harshula@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>