arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()

When turning branch instructions into NOPs, we attempt to validate the
action by comparing the old value at the call site with the opcode of
a direct relative branch instruction pointing at the old target.

However, these call sites are statically initialized to call _mcount(),
and may be redirected via a PLT entry if the module is loaded far away
from the kernel text, leading to false negatives and spurious errors.

So skip the validation if CONFIG_ARM64_MODULE_PLTS is configured.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>