bpf, arm64: Fix compile error in dummy_tramp()

dummy_tramp() uses "lr" to refer to the x30 register, but some assembler
does not recognize "lr" and reports a build failure:

/tmp/cc52xO0c.s: Assembler messages:
/tmp/cc52xO0c.s:8: Error: operand 1 should be an integer register -- `mov lr,x9'
/tmp/cc52xO0c.s:7: Error: undefined symbol lr used as an immediate value
make[2]: *** [scripts/Makefile.build:250: arch/arm64/net/bpf_jit_comp.o] Error 1
make[1]: *** [scripts/Makefile.build:525: arch/arm64/net] Error 2

So replace "lr" with "x30" to fix it.

Fixes: b2ad54e1

 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
Reported-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/bpf/20220721121319.2999259-1-xukuohai@huaweicloud.com