backports: add hex_dump_to_buffer() with int return value

In the kernel hex_dump_to_buffer() got an int return value, returning
the amount of bytes written into the buffer. This new return value is
needed by seq_hex_dump() which we also backport.

This change was done in the following commit in mainline Linux kernel:
commit 114fc1af


Author: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Date:   Thu Feb 12 15:02:29 2015 -0800

    hexdump: make it return number of bytes placed in buffer

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>