riscv: kprobes: Move branch_rs2_idx to insn.h

riscv: kprobes: Move branch_rs2_idx to insn.h

Similar to other instruction-processing macros/functions, branch_rs2_idx
should be in insn.h.

Move it into insn.h as RV_EXTRACT_RS2_REG. This new name matches the style
in insn.h.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/linux-riscv/107d4a6c1818bf169be2407b273a0483e6d55bbb.1747215274.git.namcao@linutronix.de/
[pjw@kernel.org: updated to use RV_X_MASK and to apply]
Signed-off-by: Paul Walmsley <pjw@kernel.org>