seq_buf: Make DECLARE_SEQ_BUF() usable

Using the address operator on the array doesn't work:

./include/linux/seq_buf.h:27:27: error: initialization of ‘char *’
  from incompatible pointer type ‘char (*)[128]’
  [-Werror=incompatible-pointer-types]
   27 |                 .buffer = &__ ## NAME ## _buffer,       \
      |                           ^

Apart from fixing that, we can improve DECLARE_SEQ_BUF() by using a
compound literal to define the buffer array without attaching a name
to it. This makes the macro a single statement, allowing constructs
such as:

  static DECLARE_SEQ_BUF(my_seq_buf, MYSB_SIZE);

to work as intended.

Link: https://lkml.kernel.org/r/20240116-declare-seq-buf-fix-v1-1-915db4692f32@linux.ibm.com

Cc: stable@vger.kernel.org
Acked-by: Kees Cook <keescook@chromium.org>
Fixes: dcc4e57 ("seq_buf: Introduce DECLARE_SEQ_BUF and seq_buf_str()")
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: nathanlynch

include/linux/seq_buf.h

@@ -22,9 +22,8 @@ struct seq_buf {
 };
 
 #define DECLARE_SEQ_BUF(NAME, SIZE)			\
-	char __ ## NAME ## _buffer[SIZE] = "";		\
 	struct seq_buf NAME = {				\
-		.buffer = &__ ## NAME ## _buffer,	\
+		.buffer = (char[SIZE]) { 0 },		\
 		.size = SIZE,				\
 	}