seq_buf: Export seq_buf_printf

'seq_buf' provides a very useful abstraction for writing to a string
buffer without needing to worry about it over-flowing. However even
though the API has been stable for couple of years now its still not
exported to kernel loadable modules limiting its usage.

Hence this patch proposes update to 'seq_buf.c' to mark
seq_buf_printf() which is part of the seq_buf API to be exported to
kernel loadable GPL modules. This symbol will be used in later parts
of this patch-set to simplify content creation for a sysfs attribute.

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Piotr Maziarz <piotrx.maziarz@linux.intel.com>
Cc: Cezary Rojewski <cezary.rojewski@intel.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lore.kernel.org/r/20200615124407.32596-3-vaibhav@linux.ibm.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>