ion: fix crash when alloc len is -1

If userspace passes a length between -4095 and -1 to allocate it
will pass the len != 0 check, but when len is page aligned it will
be 0.  Check len after page aligning.

Drop the warning as well, userspace shouldn't be able to trigger
a warning in the kernel.

Change-Id: I96c7142637638991f3a9af9be7cfbb50f79f3803
Signed-off-by: Colin Cross <ccross@android.com>