[PATCH blktests] Fix _get_page_size()

[Jeff Moyer <jmoyer@redhat.com>]

There is no CONFIG_PAGE_SHIFT stored in /boot/config-`uname -r` on RHEL
systems (maybe all systems?).  As a result, tests that make use of
_get_page_size() were doing the wrong thing.  For example, throtl/002
used it to calculate I/O sizes for direct IO.  Those sizes ended up not
being a multiple of the logical block size, and hence throtl/002 was
failing.

Fixes: 8eca9fa ("common/rc, scsi/011, zbd/010: introduce _page_size_equals() helper")
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

diff --git a/common/rc b/common/rc
index 20f7c7a..d60a125 100644
--- a/common/rc
+++ b/common/rc
@@ -562,13 +562,8 @@ _have_systemctl_unit() {
 	return 0
 }
 
-# Get system page size from kernel conguration
 _get_page_size() {
-	local page_shift
-
-	page_shift=$(_get_kernel_option PAGE_SHIFT)
-
-	echo $((1<< page_shift))
+	getconf PAGE_SIZE
 }
 
 # Check if the system page size matches the required size (in bytes).