arm64: mm: unexport set_memory_ro and set_memory_rw

This effectively unexports set_memory_ro and set_memory_rw functions from
commit 11d91a77 ("arm64: Add CONFIG_DEBUG_SET_MODULE_RONX support").

No module user of those is in mainline kernel and we explicitly do not want
modules to use these functions, as they i.e. RO-protect eBPF (interpreted and
JIT'ed) images from malicious modifications/bugs.

Outside of eBPF scope, I believe also other set_memory_* functions should
be unexported on arm64 due to non-existant mainline module user. Laura
mentioned that they have some uses for modules doing set_memory_*, but
none that are in mainline and it's unclear if they would ever get there.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>