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.au

Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>