ion: msm_ion: fix incorrect ion_buffer member assignment

ION_PRIV_FLAG corresponds to ion_buffer.private_flags member. Fix incorrect
assignment of ION_PRIV_FLAG_SHRINKER_FREE to ion_buffer.flags.

Change-Id: Ic24085a78c25fe9efa89a790ee80a20a5cb05fb3
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>