selftests: fix memory-hotplug test

In the memory offline test, the $ration was used with RANDOM as the
possibility to get it offlined, correct it to become the portion of
available removable memory blocks.

Also ask the tool to try to offline the next available memory block
if the attempt is unsuccessful. It will only fail if all removable
memory blocks are busy.

A nice example:
$ sudo ./test.sh
Test scope: 10% hotplug memory
	online all hot-pluggable memory in offline state:
		SKIPPED - no hot-pluggable memory in offline state
	offline 10% hot-pluggable memory in online state
	trying to offline 3 out of 28 memory block(s):
online->offline memory1
online->offline memory10
./test.sh: line 74: echo: write error: Resource temporarily unavailable
offline_memory_expect_success 10: unexpected fail
online->offline memory100
online->offline memory101
	online all hot-pluggable memory in offline state:
offline->online memory1
offline->online memory100
offline->online memory101
skip extra tests: debugfs is not mounted
$ echo $?
0

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>