riscv/mm: Add handling for VM_FAULT_SIGSEGV in mm_fault_error()

Handle VM_FAULT_SIGSEGV in the page fault path so that we correctly
kill the process and we don't BUG() the kernel.

Fixes: 07037db5 ("RISC-V: Paging and MMU")
Signed-off-by: Zhe Qiao <qiaozhe@iscas.ac.cn>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240731084547.85380-1-qiaozhe@iscas.ac.cn


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>