seq_buf: Add seq_buf_terminate() API

In the case that the seq_buf buffer needs to be printed directly, add a way
to make sure that the buffer is safe to read by forcing a nul terminating
character at the end of the string, or the last byte of the buffer if the
string has overflowed.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>