riscv: mm: Fix code comment

Fix the comment since vmalloc_fault doesn't reach
flush_tlb_fix_spurious_fault.

Signed-off-by: ShihPo Hung <shihpo.hung@sifive.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: linux-riscv@lists.infradead.org
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>