seq_buf: Add seq_buf_terminate() API

mainline inclusion
from mainline-v5.13-rc1
commit f2616c77
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8C7BS

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f2616c772c768485de18e7fcb2816bcdcd098339



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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>
Signed-off-by: Lu Jialin <lujialin4@huawei.com>