LU-18651 tests: use urandom in sanity-pfl test

LU-18651 tests:  use urandom in sanity-pfl test

With newer ZF having compression on test using dd with /dev/zero
will fail. Change the sanity-pfl test to use /dev/urandom
instead.

Change-Id: I508611fd3cee96bab623e3e12fbc1cad060b72f7
Test-Parameters: trivial fstype=zfs testlist=sanity-pfl
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58019
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>