sparc64: Fix gfp_flags setting in tsb_grow().

We should "|= more_flags" rather than "= more_flags".

Reported-by: David Rientjes <rientjes@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>