ZTS: Increase write sizes for RAIDZ/dRAID tests

Many RAIDZ/dRAID tests filled files doing millions of 100 or even
10 byte writes. It makes very little sense since we are not
micro-benchmarking syscalls or VFS layer here, while before the
blocks reach the vdev layer absolute majority of the small writes
will be aggregated. In some cases I see we spend almost as much
time creating the test files as actually running the tests. And
sometimes the tests even time out after that.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16905