riscv: kfence: Call mark_new_valid_map() for kfence_unprotect()

riscv: kfence: Call mark_new_valid_map() for kfence_unprotect()

In kfence_protect_page(), which kfence_unprotect() calls, we cannot send
IPIs to other CPUs to ask them to flush TLB. This may lead to those CPUs
spuriously faulting on a recently allocated kfence object despite it
being valid, leading to false positive use-after-free reports.

Fix this by calling mark_new_valid_map() so that the page fault handling
code path notices the spurious fault and flushes TLB then retries the
access.

Update the comment in handle_exception to indicate that
new_valid_map_cpus_check also handles kfence_unprotect() spurious
faults.

Note that kfence_protect() has the same stale TLB entries problem, but
that leads to false negatives, which is fine with kfence.

Cc: stable@vger.kernel.org
Reported-by: Yanko Kaneti <yaneti@declera.com>
Fixes: b3431a8bb336 ("riscv: Fix IPIs usage in kfence_protect_page()")
Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
Link: https://patch.msgid.link/20260303-handle-kfence-protect-spurious-fault-v2-2-f80d8354d79d@iscas.ac.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>