MIPS: Make virt_to_phys() work for all unmapped addresses.

As reported:
  This problem was discovered when doing BGP traffic with the TCP MD5 option
  activated, where the following call chain caused a crash:

   * tcp_v4_rcv
   *  tcp_v4_timewait_ack
   *   tcp_v4_send_ack -> follow stack variable rep.th
   *    tcp_v4_md5_hash_hdr
   *     tcp_md5_hash_header
   *      sg_init_one
   *       sg_set_buf
   *        virt_to_page

  I noticed that tcp_v4_send_reset uses a similar stack variable and
  also calls tcp_v4_md5_hash_hdr, so it has the same problem.

The networking core can indirectly call virt_to_phys() on stack
addresses, if this is done from PID 0, the stack will usually be in
CKSEG0, so virt_to_phys() needs to work there as well
Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: Jiang Liu <liuj97@gmail.com>
Cc: eunb.song@samsung.com
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/5220/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/io.h

@@ -118,7 +118,7 @@ static inline void set_io_port_base(unsigned long base)
  */
 static inline unsigned long virt_to_phys(volatile const void *address)
 {
-	return (unsigned long)address - PAGE_OFFSET + PHYS_OFFSET;
+	return __pa(address);
 }
 
 /*

arch/mips/include/asm/page.h

@@ -46,7 +46,6 @@
 #endif /* CONFIG_MIPS_HUGE_TLB_SUPPORT */
 
 #include <linux/pfn.h>
-#include <asm/io.h>
 
 extern void build_clear_page(void);
 extern void build_copy_page(void);
@@ -151,6 +150,7 @@ typedef struct { unsigned long pgprot; } pgprot_t;
     ((unsigned long)(x) - PAGE_OFFSET + PHYS_OFFSET)
 #endif
 #define __va(x)		((void *)((unsigned long)(x) + PAGE_OFFSET - PHYS_OFFSET))
+#include <asm/io.h>
 
 /*
  * RELOC_HIDE was originally added by 6007b903dfe5f1d13e0c711ac2894bdd4a61b1ad