NFSv4: Clear NFS_CAP_OPEN_XOR and NFS_CAP_DELEGTIME if not supported

_nfs4_server_capabilities() should clear capabilities that are not
supported by the server.

Fixes: d2a00cce ("NFSv4: Detect support for OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>