LU-14321 tests: create PFL file in sanityn 51b

LU-14321 tests: create PFL file in sanityn 51b

sanityn test 51b was modified to integrate statx API in
Lustre version 2.13.54.  When we run version interop testing
with servers less than 2.13.54 and later clients, the test
will fail.

We should modify the test to create a PFL file without the
'extension-size' lfs setstripe option which will allow this
test to run with servers less than 2.13.54.

Fixes: 3f7853b31ef6 ("LU-10934 llite: integrate statx() API with Lustre")
Test-Parameters: trivial
Test-Parameters: serverversion=2.12.7 serverdistro=el7.9 env=ONLY=51b testlist=sanityn
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Change-Id: Ic3feb72771aa2db050b792159175624260e71f5b
Reviewed-on: https://review.whamcloud.com/44027
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>