input: synaptics: put offset checks under mutex.

Place file offset validity checks under mutex.

BUG: 33555878
BUG: 33002026

Change-Id: I1945cfc8af7d1a310ae0d7bbb85002d4c448f30b
Signed-off-by: Andrew Chant <achant@google.com>
Git-repo: https://android.googlesource.com/kernel/msm


Git-commit: e1fb1600fc222337989e3084d68df929882deae5
Resolved minor merge conflict
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>