input: synaptics_dsx_2.6: protect tmpbuf allocation

Protect tmpbuf from concurrent access by mutex.
BUG: 33555878
BUG: 33002026

Git-repo: https://android.googlesource.com/kernel/msm


Git-commit: e6430a4da1fb0212a546379eadbe986f629c3ae9
Change-Id: Ia7eeb59ca7b626f416e2298b4b9ffd960fe909e4
Signed-off-by: Andrew Chant <achant@google.com>
[shjain@codeaurora.org: apply the similar code change
to synaptics v2.6 driver, as it was not applying cleanly.]
Signed-off-by: Shantanu Jain <shjain@codeaurora.org>