seq_buf: Add seq_buf_can_fit() helper function

Add a seq_buf_can_fit() helper function that removes the possible mistakes
of comparing the seq_buf length plus added data compared to the size of
the buffer.

Link: http://lkml.kernel.org/r/20141118164025.GL23958@pathway.suse.cz

Reviewed-by: Petr Mladek <pmladek@suse.cz>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>