The seq_buf KUnit suite does not exercise seq_buf_putmem_hex().

Add one test for the len > 8 chunking path and one overflow test
where a later chunk no longer fits in the buffer.

Export seq_buf_putmem_hex() as well so SEQ_BUF_KUNIT_TEST=m links
cleanly. Without the export, modpost reports seq_buf_putmem_hex as
undefined when seq_buf_kunit is built as a module.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: David Gow <david@davidgow.net>
Link: https://patch.msgid.link/20260408202351.21829-1-shuvampandey1@gmail.com
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuvam Pandey <shuvampandey1@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Add KUnit tests for the seq_buf API to ensure its correctness and
prevent future regressions, covering the following functions:
- seq_buf_init()
- DECLARE_SEQ_BUF()
- seq_buf_clear()
- seq_buf_puts()
- seq_buf_putc()
- seq_buf_printf()
- seq_buf_get_buf()
- seq_buf_commit()

$ tools/testing/kunit/kunit.py run seq_buf
=================== seq_buf (9 subtests) ===================
[PASSED] seq_buf_init_test
[PASSED] seq_buf_declare_test
[PASSED] seq_buf_clear_test
[PASSED] seq_buf_puts_test
[PASSED] seq_buf_puts_overflow_test
[PASSED] seq_buf_putc_test
[PASSED] seq_buf_printf_test
[PASSED] seq_buf_printf_overflow_test
[PASSED] seq_buf_get_buf_commit_test
===================== [PASSED] seq_buf =====================

Link: https://lore.kernel.org/r/20250717085156.work.363-kees@kernel.org
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Kees Cook <kees@kernel.org>