seq_buf: Use size_t for len in seq_buf_puts()

Jann Horn points out that we're using unsigned int for len in
seq_buf_puts(), which could potentially overflow if we're passed a
UINT_MAX sized string.

The rest of the code already uses size_t, so we should also use that
in seq_buf_puts() to avoid any issues.

Link: http://lkml.kernel.org/r/20181019042109.8064-2-mpe@ellerman.id.auSuggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

lib/seq_buf.c

@@ -140,7 +140,7 @@ int seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary)
  */
 int seq_buf_puts(struct seq_buf *s, const char *str)
 {
-	unsigned int len = strlen(str);
+	size_t len = strlen(str);
 
 	WARN_ON(s->size == 0);