mips: fix a compilation error.

From: xiaolinkui <[email protected]>

Commit 4d5b3bdc0ecb ("MIPS: Fix a warning for virt_to_page") will trigger a compilation error as follows with kernel v5.17-rc8:
(CONFIG_RDMA_SIW=y)

In file included from ./arch/mips/include/asm/page.h:270,

                 from ./arch/mips/include/asm/io.h:29,
                 from ./arch/mips/include/asm/mmiowb.h:5,
                 from ./include/linux/spinlock.h:64,
                 from ./include/linux/wait.h:9,
                 from ./include/linux/net.h:19,
                 from drivers/infiniband/sw/siw/siw_qp_tx.c:8:

drivers/infiniband/sw/siw/siw_qp_tx.c: In function ‘siw_tx_hdt’:
./arch/mips/include/asm/page.h:255:53: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
255 | #define virt_to_pfn(kaddr) PFN_DOWN(virt_to_phys((void *)(kaddr)))
| ^
./include/asm-generic/memory_model.h:18:41: note: in definition of macro ‘__pfn_to_page’
18 | #define __pfn_to_page(pfn) (mem_map + ((pfn) - ARCH_PFN_OFFSET))
| ^~~
./arch/mips/include/asm/page.h:255:31: note: in expansion of macro ‘PFN_DOWN’

  255 | #define virt_to_pfn(kaddr)    PFN_DOWN(virt_to_phys((void *)(kaddr)))
      |                               ^~~~~~~~

./arch/mips/include/asm/page.h:256:41: note: in expansion of macro ‘virt_to_pfn’
256 | #define virt_to_page(kaddr) pfn_to_page(virt_to_pfn(kaddr))
| ^~~~~~~~~~~
drivers/infiniband/sw/siw/siw_qp_tx.c:538:23: note: in expansion of macro ‘virt_to_page’

  538 |     page_array[seg] = virt_to_page(va & PAGE_MASK);
      |                       ^~~~~~~~~~~~

---
 arch/mips/include/asm/page.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 96bc798c1ec1..0f986f4ad4f0 100644
@@ -252,7 +252,7 @@ static inline int pfn_valid(unsigned long pfn)
 
 #endif
 
-#define virt_to_pfn(kaddr)   	PFN_DOWN(virt_to_phys((void *)(kaddr)))
+#define virt_to_pfn(kaddr)   	PFN_DOWN(__pa(kaddr))
 #define virt_to_page(kaddr)	pfn_to_page(virt_to_pfn(kaddr))
 
 extern bool __virt_addr_valid(const volatile void *kaddr);

2.17.1