Fix off-by-one error with binderThroughputTest -s

The -s option to binderThroughputTest allows the user
to set the size of the payload being sent during the
test. Due to an off-by-one error, the size was actually
4 bytes less than specified. Fixed.

Test: verified correct size
Bug: 65052700
Change-Id: I884fe08c113c2ca63a6f328741fb0914799e9eb7
Signed-off-by: Todd Kjos <tkjos@google.com>