riscv: mm: Add pfn_to_kaddr() implementation

The pfn_to_kaddr() function is used by KASAN's memory hotplugging
path. Add the missing function to the RISC-V port, so that it can be
built with MHP and CONFIG_KASAN.
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240605114100.315918-6-bjorn@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/page.h

@@ -188,6 +188,11 @@ extern phys_addr_t __phys_addr_symbol(unsigned long x);
 
 unsigned long kaslr_offset(void);
 
+static __always_inline void *pfn_to_kaddr(unsigned long pfn)
+{
+	return __va(pfn << PAGE_SHIFT);
+}
+
 #endif /* __ASSEMBLY__ */
 
 #define virt_addr_valid(vaddr)	({						\