gpu: ion: Fix race between ion_import and ion_free

If preemted during ion_free after the refcount is updated but
before the handle can be removed from the rb_tree, import
might find that handle in the tree and try to reuse it
when execution returns to free, the handle will be cleaned
up leaving the caller of import with a corrupt handle.
This patch modifies the locking to protect agains this race.

Change-Id: I31d18cc6398f0ca18e05cd919e2bcf86fa18d568
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>