When enabling both CONFIG_KFENCE and CONFIG_DEBUG_SG, I get the
following backtraces when running LongArch kernels.

[    2.496257] kernel BUG at include/linux/scatterlist.h:187!
...
[    2.501925] Call Trace:
[    2.501950] [<9000000004ad59c4>] sg_init_one+0xac/0xc0
[    2.502204] [<9000000004a438f8>] do_test_kpp+0x278/0x6e4
[    2.502353] [<9000000004a43dd4>] alg_test_kpp+0x70/0xf4
[    2.502494] [<9000000004a41b48>] alg_test+0x128/0x690
[    2.502631] [<9000000004a3d898>] cryptomgr_test+0x20/0x40
[    2.502775] [<90000000041b4508>] kthread+0x138/0x158
[    2.502912] [<9000000004161c48>] ret_from_kernel_thread+0xc/0xa4

The backtrace is always similar but not exactly the same. It is always
triggered from cryptomgr_test, but not always from the same test.

Analysis shows that with CONFIG_KFENCE active, the address returned from
kmalloc() and friends is not always below vm_map_base. It is allocated
by kfence_alloc() which at least sometimes seems to get its memory from
an address space above vm_map_base. This causes __virt_addr_valid() to
return false for the affected objects.

Let __virt_addr_valid() return 1 for kfence pool addresses, this make
virt_addr_valid()/__virt_addr_valid() work with KFENCE.
Reported-by: Guenter Roeck <linux@roeck-us.net>
Suggested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>