samples/kprobes: add a new module parameter

Add a new module parameter which can be used as the symbol name.

Without this patch, we can only test the "_do_fork" function with this
kernel module.  With this patch, the module becomes more flexible; we
can test any functions with this module with

	# insmod kprobe_example.ko symbol="xxx"

Link: http://lkml.kernel.org/r/1463535417-29637-1-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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>