Change the CHECK failure into function failure.

Previously we have CHECK in WriteInternal function to observe short
writing. However it turns out short write can happen according to the
bug report.

To prevent app from crashing due to CHECK failure, the CL removes the
CHECK and let WriteInternal return a failure value.

Bug: 37561460
Test: libappfuse_tests, manually re-wrote the return value of write()
      and checked logcat.
Change-Id: I6a1e233c3ddb8eb68f59e7c606ad0459b5ca2c6e