md: replace deprecated strncpy with memcpy

md: replace deprecated strncpy with memcpy

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

There are three such strncpy uses that this patch addresses:

The respective destination buffers are:
1) mddev->clevel
2) clevel
3) mddev->metadata_type

We expect mddev->clevel to be NUL-terminated due to its use with format
strings:
|       ret = sprintf(page, "%s\n", mddev->clevel);

Furthermore, we can see that mddev->clevel is not expected to be
NUL-padded as `md_clean()` merely set's its first byte to NULL -- not
the entire buffer:
|       static void md_clean(struct mddev *mddev)
|       {
|       	mddev->array_sectors = 0;
|       	mddev->external_size = 0;
|               ...
|       	mddev->level = LEVEL_NONE;
|       	mddev->clevel[0] = 0;
|               ...

A suitable replacement for this instance is `memcpy` as we know the
number of bytes to copy and perform manual NUL-termination at a
specified offset. This really decays to just a byte copy from one buffer
to another. `strscpy` is also a considerable replacement but using
`slen` as the length argument would result in truncation of the last
byte unless something like `slen + 1` was provided which isn't the most
idiomatic strscpy usage.

For the next case, the destination buffer `clevel` is expected to be
NUL-terminated based on its usage within kstrtol() which expects
NUL-terminated strings. Note that, in context, this code removes a
trailing newline which is seemingly not required as kstrtol() can handle
trailing newlines implicitly. However, there exists further usage of
clevel (or buf) that would also like to have the newline removed. All in
all, with similar reasoning to the first case, let's just use memcpy as
this is just a byte copy and NUL-termination is handled manually.

The third and final case concerning `mddev->metadata_type` is more or
less the same as the other two. We expect that it be NUL-terminated
based on its usage with seq_printf:
|       seq_printf(seq, " super external:%s",
|       	   mddev->metadata_type);
... and we can surmise that NUL-padding isn't required either due to how
it is handled in md_clean():
|       static void md_clean(struct mddev *mddev)
|       {
|       ...
|       mddev->metadata_type[0] = 0;
|       ...

So really, all these instances have precisely calculated lengths and
purposeful NUL-termination so we can just use memcpy to remove ambiguity
surrounding strncpy.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230925-strncpy-drivers-md-md-c-v1-1-2b0093b89c2b@google.com