This patch implements CONFIG_DEBUG_VIRTUAL to do additional checks on
virt_to_phys and __pa_symbol calls. virt_to_phys used for linear mapping
check, and __pa_symbol used for kernel symbol check. In current RISC-V,
kernel image maps to linear mapping area. If CONFIG_DEBUG_VIRTUAL is
disable, these two functions calculate the offset on the address feded
directly without any checks.

The result of test_debug_virtual as follows:

[    0.358456] ------------[ cut here ]------------
[    0.358738] virt_to_phys used for non-linear address: (____ptrval____) (0xffffffd000000000)
[    0.359174] WARNING: CPU: 0 PID: 1 at arch/riscv/mm/physaddr.c:16 __virt_to_phys+0x3c/0x50
[    0.359409] Modules linked in:
[    0.359630] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.5.0-rc3-00002-g5133c5c0ca13 #57
[    0.359861] epc: ffffffe000253d1a ra : ffffffe000253d1a sp : ffffffe03aa87da0
[    0.360019]  gp : ffffffe000ae03b0 tp : ffffffe03aa88000 t0 : ffffffe000af2660
[    0.360175]  t1 : 0000000000000064 t2 : 00000000000000b7 s0 : ffffffe03aa87dc0
[    0.360330]  s1 : ffffffd000000000 a0 : 000000000000004f a1 : 0000000000000000
[    0.360492]  a2 : 0000000000000000 a3 : 0000000000000000 a4 : ffffffe000a84358
[    0.360672]  a5 : 0000000000000000 a6 : 0000000000000000 a7 : 0000000000000000
[    0.360876]  s2 : ffffffe000ae0600 s3 : ffffffe00000fc7c s4 : ffffffe0000224b0
[    0.361067]  s5 : ffffffe000030890 s6 : ffffffe000022470 s7 : 0000000000000008
[    0.361267]  s8 : ffffffe0000002c4 s9 : ffffffe000ae0640 s10: ffffffe000ae0630
[    0.361453]  s11: 0000000000000000 t3 : 0000000000000000 t4 : 000000000001e6d0
[    0.361636]  t5 : ffffffe000ae0a18 t6 : ffffffe000aee54e
[    0.361806] status: 0000000000000120 badaddr: 0000000000000000 cause: 0000000000000003
[    0.362056] ---[ end trace aec0bf78d4978122 ]---
[    0.362404] PA: 0xfffffff080200000 for VA: 0xffffffd000000000
[    0.362607] PA: 0x00000000baddd2d0 for VA: 0xffffffe03abdd2d0
Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Tested-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>