seq_buf: Add seq_buf_do_printk() helper

Sometimes we use seq_buf to format a string buffer, which
we then pass to printk(). However, in certain situations
the seq_buf string buffer can get too big, exceeding the
PRINTKRB_RECORD_MAX bytes limit, and causing printk() to
truncate the string.

Add a new seq_buf helper. This helper prints the seq_buf
string buffer line by line, using \n as a delimiter,
rather than passing the whole string buffer to printk()
at once.

Link: https://lkml.kernel.org/r/20230415100110.1419872-1-senozhatsky@chromium.org



Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>