Various fixes in bspatch and File class.

bspatch should add diff block and old file.

Fix crash on free(new_buf).

open() in bionic needs mode to be set if O_CREAT.

Fix typo in Seek().

Wrong GetSize() for regular file.

Bug: 24478450
Test: Delta update passes filesystem verification on target partition.

Change-Id: Ie53b7cf2b7467fb2cf78762d41e7f65674afa969