seq_buf: Make DECLARE_SEQ_BUF() usable

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: dcc4e5728eea ("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>