staging: ion: msm: Apply 2c155709 to msm_ion_custom_ioctl

Commit 2c155709 ("staging: android: ion: fix ION_IOC_{MAP,SHARE}
use-after-free") removed the definition of ion_handle_get_by_id,
replacing it with a lockless version. Update msm_ion_custom_ioctl with
this new definition and locking scheme for completeness (it appears that
the ioctl does not suffer from the issue outlined by Greg Hackmann).

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>