ZTS: Fix fault_limits timeouts

fault_limits would often hit the 10min timeout and be killed on Fedora
41-42. Investigation showed that the 'fill_fs' portion of the test,
which would fill the pool with junk data before vdev replacement, was
writing highly compressible data (~126x), which would have taxed the
CPUs, potentially causing the timeout.

The fix is to write random data and reduce the number of writes.
This has an added benefit that more real data being is written to the
pool (~1GB) vs the old way (~300-400MB). It also speeds up the test.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17709