samples/kretprobe: fix the wrong type

samples/kretprobe: fix the wrong type

The regs_return_value() returns "unsigned long" or "long" value.  But the
retval is int type now, it may cause overflow, the log may becomes:

    [ 2911.078869] do_brk returned -2003877888 and took 4620 ns to execute

This patch converts the retval to "unsigned long" type, and fixes the
overflow issue.

Link: http://lkml.kernel.org/r/1464143083-3877-4-git-send-email-shijie.huang@arm.com
Signed-off-by: Huang Shijie <shijie.huang@arm.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Steve Capper <steve.capper@arm.com>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>