Documentation: kdump: Add va_kernel_pa_offset for RISCV64

RISC-V Linux exports "va_kernel_pa_offset" in vmcoreinfo to help
Crash-utility translate the kernel virtual address correctly.

Here adds the definition of "va_kernel_pa_offset".

Fixes: 3335068 ("riscv: Use PUD/P4D/PGD pages for the linear mapping")
Link: https://lore.kernel.org/linux-riscv/[email protected]/
Signed-off-by: Song Shuai <[email protected]>
Reviewed-by: Alexandre Ghiti <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Cc: [email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: Song Shuai

Documentation/admin-guide/kdump/vmcoreinfo.rst

@@ -624,3 +624,9 @@ Used to get the correct ranges:
   * VMALLOC_START ~ VMALLOC_END : vmalloc() / ioremap() space.
   * VMEMMAP_START ~ VMEMMAP_END : vmemmap space, used for struct page array.
   * KERNEL_LINK_ADDR : start address of Kernel link and BPF
+
+va_kernel_pa_offset
+-------------------
+
+Indicates the offset between the kernel virtual and physical mappings.
+Used to translate virtual to physical addresses.