firmware: tegra: Reduce stack usage

Building the bpmp-debugfs driver for Arm results in a warning for stack usage:

drivers/firmware/tegra/bpmp-debugfs.c:321:16: error: stack frame size of 1224 bytes in function 'bpmp_debug_store' [-Werror,-Wframe-larger-than=]
static ssize_t bpmp_debug_store(struct file *file, const char __user *buf,

It should be possible to rearrange the code to not require two separate
buffers for the file name, but the easiest workaround is to use dynamic
allocation.

Fixes: 5e37b9c1 ("firmware: tegra: Add support for in-band debug")
Link: https://lore.kernel.org/all/20201204193714.3134651-1-arnd@kernel.org/Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[treding@nvidia.com: consistently return NULL on failure]
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/firmware/tegra/bpmp-debugfs.c

@@ -74,28 +74,36 @@ static void seqbuf_seek(struct seqbuf *seqbuf, ssize_t offset)
 static const char *get_filename(struct tegra_bpmp *bpmp,
 				const struct file *file, char *buf, int size)
 {
-	char root_path_buf[512];
-	const char *root_path;
-	const char *filename;
+	const char *root_path, *filename = NULL;
+	char *root_path_buf;
 	size_t root_len;
 
+	root_path_buf = kzalloc(512, GFP_KERNEL);
+	if (!root_path_buf)
+		goto out;
+
 	root_path = dentry_path(bpmp->debugfs_mirror, root_path_buf,
 				sizeof(root_path_buf));
 	if (IS_ERR(root_path))
-		return NULL;
+		goto out;
 
 	root_len = strlen(root_path);
 
 	filename = dentry_path(file->f_path.dentry, buf, size);
-	if (IS_ERR(filename))
-		return NULL;
+	if (IS_ERR(filename)) {
+		filename = NULL;
+		goto out;
+	}
 
-	if (strlen(filename) < root_len ||
-			strncmp(filename, root_path, root_len))
-		return NULL;
+	if (strlen(filename) < root_len || strncmp(filename, root_path, root_len)) {
+		filename = NULL;
+		goto out;
+	}
 
 	filename += root_len;
 
+out:
+	kfree(root_path_buf);
 	return filename;
 }