ZTS: fix send-c_stream_size_estimate

The test could fail when attempting to write to a newly created
volume which was missing its device node. Resolve the issue by
calling block_device_wait() which blocks until udev creates the
needed entry.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7276
Closes #7277