LU-12219 obdfilter: changes PAGE_SIZE variable

LU-12219 obdfilter: changes PAGE_SIZE variable

obdfilter-survey uses PAGE_SIZE in KBytes. After LU-11597
PAGE_SIZE exported from test-framework.sh in bytes. So it confuses
obdfilter-survey and lead to error:
/usr/bin/obdfilter-survey: line 509: size * 1024 / (actual_rsz * thr):
division by 0 (error token is ")")

Patch changes the name to PAGE_SIZE_KB.

Fixes: f602b5ec7f4 ("LU-11597 tests: fix O_DIRECT test usage for ARM")
Test-Parameters: trivial testlist=obdfilter-survey
Signed-off-by: Alexander Boyko <c17825@cray.com>
Cray-bug-id: LUS-7214
Change-Id: Ie8be852c9634569c59a770ba49c3d1c36f53fdb2
Reviewed-on: https://review.whamcloud.com/34754
Reviewed-by: Elena Gryaznova <c17455@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>