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 <[email protected]>
Cc: [email protected]
Cc: Jiang Liu <[email protected]>
Cc: [email protected]
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/5220/
Signed-off-by: Ralf Baechle <[email protected]>

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