nvdimm/btt: replace deprecated strncpy with strscpy

Found with grep.

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

We expect super->signature to be NUL-terminated based on its usage with
memcmp against a NUL-term'd buffer:
btt_devs.c:
253 | if (memcmp(super->signature, BTT_SIG, BTT_SIG_LEN) != 0)
btt.h:
13  | #define BTT_SIG "BTT_ARENA_INFO\0"

NUL-padding is not required as `super` is already zero-allocated:
btt.c:
985 | super = kzalloc(sizeof(struct btt_sb), GFP_NOIO);
... rendering any additional NUL-padding superfluous.

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

Let's also use the more idiomatic strscpy usage of (dest, src,
sizeof(dest)) instead of (dest, src, XYZ_LEN) for buffers that the
compiler can determine the size of. This more tightly correlates the
destination buffer to the amount of bytes copied.

Side note, this pattern of memcmp() on two NUL-terminated strings should
really be changed to just a strncmp(), if i'm not mistaken? I see
multiple instances of this pattern in this system:

|       if (memcmp(super->signature, BTT_SIG, BTT_SIG_LEN) != 0)
|               return false;

where BIT_SIG is defined (weirdly) as a double NUL-terminated string:

|       #define BTT_SIG "BTT_ARENA_INFO\0"

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: KSPP#90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231019-strncpy-drivers-nvdimm-btt-c-v2-1-366993878cf0@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: JustinStitt

drivers/nvdimm/btt.c

@@ -986,7 +986,7 @@ static int btt_arena_write_layout(struct arena_info *arena)
 	if (!super)
 		return -ENOMEM;
 
-	strncpy(super->signature, BTT_SIG, BTT_SIG_LEN);
+	strscpy(super->signature, BTT_SIG, sizeof(super->signature));
 	export_uuid(super->uuid, nd_btt->uuid);
 	export_uuid(super->parent_uuid, parent_uuid);
 	super->flags = cpu_to_le32(arena->flags);