android: binder: enable preemption for vfree

New changes to vfree makes it a sleepable function
which makes it an offense to call vfree from the
binder_deferred_func (via binder_deferred_release).
binder_deferred_func disables preemption for reducing
lock latencies, so to avoid the problem enable preemption
for the duration of vfree execution. vfree_atomic can't be
used as binder unmaps the buffer before doing free, and
vfree_atomic dereferences the to-be-freed address.

Change-Id: I093b37a6d049c7cbb599aeb05012b189b45b587b
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>