riscv: uprobes: Add missing fence.i after building the XOL buffer

The XOL (execute out-of-line) buffer is used to single-step the
replaced instruction(s) for uprobes. The RISC-V port was missing a
proper fence.i (i$ flushing) after constructing the XOL buffer, which
can result in incorrect execution of stale/broken instructions.

This was found running the BPF selftests "test_progs:
uprobe_autoattach, attach_probe" on the Spacemit K1/X60, where the
uprobes tests randomly blew up.

Reviewed-by: Guo Ren <guoren@kernel.org>
Fixes: 7478408 ("riscv: Add uprobes supported")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20250419111402.1660267-2-bjorn@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: bjorn-rivos

arch/riscv/kernel/probes/uprobes.c

@@ -167,6 +167,7 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
 	/* Initialize the slot */
 	void *kaddr = kmap_atomic(page);
 	void *dst = kaddr + (vaddr & ~PAGE_MASK);
+	unsigned long start = (unsigned long)dst;
 
 	memcpy(dst, src, len);
 
@@ -176,13 +177,6 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
 		*(uprobe_opcode_t *)dst = __BUG_INSN_32;
 	}
 
+	flush_icache_range(start, start + len);
 	kunmap_atomic(kaddr);
-
-	/*
-	 * We probably need flush_icache_user_page() but it needs vma.
-	 * This should work on most of architectures by default. If
-	 * architecture needs to do something different it can define
-	 * its own version of the function.
-	 */
-	flush_dcache_page(page);
 }