Input: synaptics: check input, prevent sysfs races

concurrent sysfs calls on the fw updater can cause
ugly race conditions.  Return EBUSY on concurrent sysfs calls.

For sysfs calls which generate deferred work, prevent
the deferred work from running concurrently with other
sysfs calls.

Also check that ext_data_source is appropriately sized
and allocated, based on a patch by
Gengjia Chen (chengjia4574@gmail.com).

Change-Id:I5bbe4992f3fd2d23db288296eaeb61f5831098e9
Signed-off-by: Andrew Chant <achant@google.com>
Bug: 30799828
Bug: 31252388
Git-commit: 287ce2ccfefe68067c1f9f5175b6664bf7397fe6
Git-repo: https://android.googlesource.com/kernel/msm.git


[vevekv@codeaurora.org: resolve merge conflicts]
Signed-off-by: Vevek Venkatesan <vevekv@codeaurora.org>