LU-18468 tests: write PAGE_SIZE for sanity-flr/211

LU-18468 tests: write PAGE_SIZE for sanity-flr/211

Do not write smaller than PAGE_SIZE (=64KiB on aarch64/ppc64) when
using "dd oflag=direct" for sanity-flr test_211.  Otherwise, this
will fail on clients/servers that do not support unaligned direct IO.

Test-Parameters: trivial
Test-Parameters: testlist=sanity-flr env=ONLY=211 clientarch=aarch64 clientdistro=el9.3 serverversion=2.14
Fixes: b5db891cad ("LU-18468 llapi: always truncate mirrors on resync")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ia102b99e1303bb24114e58ecc129fe27a198135e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58122
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>