alloc_tag: Tighten file permissions on /proc/allocinfo

The /proc/allocinfo file exposes a tremendous about of information about
kernel build details, memory allocations (obviously), and potentially even
image layout (due to ordering).  As this is intended to be consumed by
system owners (like /proc/slabinfo), use the same file permissions as
there: 0400.

Link: https://lkml.kernel.org/r/20240425200844.work.184-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>