riscv: extable: use `ex` for `exception_table_entry`

The var name "fixup" is a bit confusing, since this is a
exception_table_entry. Use "ex" instead  to refer to an entire entry.
In subsequent patches we'll use `fixup` to refer to the fixup
field specifically.

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