kmemleak: Printing of the objects hex dump

Introducing printing of the objects hex dump to the seq file.
The number of lines to be printed is limited to HEX_MAX_LINES
to prevent seq file spamming. The actual number of printed
bytes is less than or equal to (HEX_MAX_LINES * HEX_ROW_SIZE).

(slight adjustments by Catalin Marinas)

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@mail.by>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>