fs/smb/client: add verbose error logging for UNC parsing

Add cifs_dbg(VFS, ...) statements to smb3_parse_devname() to provide
explicit feedback when parsing fails. Currently, the function returns
-EINVAL silently, making it difficult to debug mount failures caused
by malformed paths or missing share names.

Signed-off-by: Fredric Cover <FredTheDude@proton.me>
Acked-by: Henrique Carvalho <[2]henrique.carvalho@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Fredric Cover

fs/smb/client/fs_context.c

@@ -662,13 +662,17 @@ smb3_parse_devname(const char *devname, struct smb3_fs_context *ctx)
 
 	/* make sure we have a valid UNC double delimiter prefix */
 	len = strspn(devname, delims);
-	if (len != 2)
+	if (len != 2) {
+		cifs_dbg(VFS, "UNC: path must begin with // or \\\\\n");
 		return -EINVAL;
+	}
 
 	/* find delimiter between host and sharename */
 	pos = strpbrk(devname + 2, delims);
-	if (!pos)
+	if (!pos) {
+		cifs_dbg(VFS, "UNC: missing delimiter between hostname and share name\n");
 		return -EINVAL;
+	}
 
 	/* record the server hostname */
 	kfree(ctx->server_hostname);
@@ -681,8 +685,10 @@ smb3_parse_devname(const char *devname, struct smb3_fs_context *ctx)
 
 	/* now go until next delimiter or end of string */
 	len = strcspn(pos, delims);
-	if (!len)
+	if (!len) {
+		cifs_dbg(VFS, "UNC: missing share name\n");
 		return -EINVAL;
+	}
 
 	/* move "pos" up to delimiter or NULL */
 	pos += len;