ZTS: Fix send_partial_dataset.ksh

The send_partial_dataset test verifies that partial send streams
can be resumed. This test may occasionally fail with a "token is
corrupt" error if the mess_send_file truncates a send stream
below the size of the DRR_BEGIN record. Update this function to
set a minimum size to ensure there is at least an intact DDR_BEGIN
record which allows for the receiving dataset to be created.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13177