Reduce size of zvol and enforce 4k blocksize in zvol tests

32-bit builders in the buildbot are having trouble completing
their ENOSPC testing in less than the timeout. Reduce the
zvol size and use a 4k block size to reduce read-modify-writes
which are particularly expensive on 32-bit systems due to the
reduced maximum ARC size.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kash Pande <kash@tripleback.net>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5845