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

In the Linux kernel, the following vulnerability has been resolved: 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.