wifi: iwlwifi: mld: Work around Clang loop unrolling bug

The nested loop in iwl_mld_send_proto_offload() confuses Clang into
thinking there could be a final loop iteration past the end of the
"nsc" array (which is only 4 entries). The FORTIFY checking in memcmp()
(via ipv6_addr_cmp()) notices this (due to the available bytes in the
out-of-bounds position of &nsc[4] being 0), and errors out, failing
the build. For some reason (likely due to architectural loop unrolling
configurations), this is only exposed on ARM builds currently. Due to
Clang's lack of inline tracking[1], the warning is not very helpful:

include/linux/fortify-string.h:719:4: error: call to '__read_overflow' declared with 'error' attribute: detected read beyond size of object (1st parameter)
719 | __read_overflow();
| ^
1 error generated.

But this was tracked down to iwl_mld_send_proto_offload()'s
ipv6_addr_cmp() call.

An upstream Clang bug has been filed[2] to track this. For now fix the
build by explicitly bounding the inner loop by "n_nsc", which is what
"c" is already limited to.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://github.com/ClangBuiltLinux/linux/issues/2076
Link: https://github.com/llvm/llvm-project/pull/73552 [1]
Link: https://github.com/llvm/llvm-project/issues/136603 [2]
Link: https://lore.kernel.org/r/20250421204153.work.935-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>