riscv: Use flush_icache_mm for flush_icache_user_range

The only call path is:

__access_remote_vm -> copy_to_user_page -> flush_icache_user_range

Seems it's ok to use flush_icache_mm instead of flush_icache_all and
it could reduce flush_icache_all called on other harts.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
[Palmer: git-am wouldn't apply the patch, I did so manually]
Fixes: 08f051ed ("RISC-V: Flush I$ when making a dirty page executable")
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>