seq_file: provide an analogue of print_hex_dump()

This introduces a new helper and switches current users to use it.  All
patches are compiled tested. kmemleak is tested via its own test suite.

This patch (of 6):

The new seq_hex_dump() is a complete analogue of print_hex_dump().

We have few users of this functionality already. It allows to reduce their
codebase.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Joe Perches <joe@perches.com>
Cc: Tadeusz Struk <tadeusz.struk@intel.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Ingo Tuchscherer <ingo.tuchscherer@de.ibm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>