Fixes coccicheck warning:

/arch/sparc/mm/srmmu.c:354:42-43: WARNING comparing pointer to 0

Avoid pointer type value compared to 0.
Reported-by: NTosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: NKaixu Xia <kaixuxia@tencent.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>