NFSv4: Clear the NFS_CAP_FS_LOCATIONS flag if it is not set

_nfs4_server_capabilities() is expected to clear any flags that are not
supported by the server.

Fixes: 8a59bb93 ("NFSv4 store server support for fs_location attribute")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>