riscv: extable: make fixup_exception() return bool

The return values of fixup_exception() and riscv_bpf_fixup_exception()
represent a boolean condition rather than an error code, so it's better
to return `bool` rather than `int`.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>