firmware: tegra: bpmp: Replace deprecated strncpy() with strscpy_pad()

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

We should prefer more robust and less ambiguous string interfaces.

It seems like the filename stored at `namevirt` is expected to be
NUL-terminated.

A suitable replacement is `strscpy_pad` due to the fact that it
guarantees NUL-termination on the destination buffer whilst maintaining
the NUL-padding behavior that strncpy provides.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
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/20230913-strncpy-drivers-firmware-tegra-bpmp-debugfs-c-v1-1-828b0a8914b5@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: JustinStitt

drivers/firmware/tegra/bpmp-debugfs.c

@@ -610,7 +610,7 @@ static int debugfs_show(struct seq_file *m, void *p)
 	}
 
 	len = strlen(filename);
-	strncpy(namevirt, filename, namesize);
+	strscpy_pad(namevirt, filename, namesize);
 
 	err = mrq_debugfs_read(bpmp, namephys, len, dataphys, datasize,
 			       &nbytes);
@@ -661,7 +661,7 @@ static ssize_t debugfs_store(struct file *file, const char __user *buf,
 	}
 
 	len = strlen(filename);
-	strncpy(namevirt, filename, namesize);
+	strscpy_pad(namevirt, filename, namesize);
 
 	if (copy_from_user(datavirt, buf, count)) {
 		err = -EFAULT;