ion: WARN when the handle kmap_cnt is going to wrap around

There are certain client bugs (double unmap, for example) that can cause
the handle->kmap_cnt (an unsigned int) to wrap around from zero. This
causes problems when the handle is destroyed because we have:

        while (handle->kmap_cnt)
                ion_handle_kmap_put(handle);

which takes a long time to complete when kmap_cnt starts at ~0 and can
result in a watchdog timeout.

WARN and bail when kmap_cnt is about to wrap around from zero.

Change-Id: I01cb09ad09b81c1d0e02e82333a14af6a08a92e9
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>