PEL: Rename Section::flattenedSize

cppcheck was complaining about how flattenedSize() was in both the base
class Section and all of its derived classes even though it wasn't
virtual.

Change-Id: I5cce8c6534e696f30c2c6141a50665d11a13eff0
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/extensions/openpower-pels/extended_user_data.cpp b/extensions/openpower-pels/extended_user_data.cpp
index 97ba86b..e685b13 100644
--- a/extensions/openpower-pels/extended_user_data.cpp
+++ b/extensions/openpower-pels/extended_user_data.cpp
@@ -110,10 +110,9 @@
 bool ExtendedUserData::shrink(size_t newSize)
 {
     // minimum size is 8B header + 4B of fields + 4B of data
-    if ((newSize < flattenedSize()) &&
-        (newSize >= (Section::flattenedSize() + 8)))
+    if ((newSize < flattenedSize()) && (newSize >= (Section::headerSize() + 8)))
     {
-        auto dataSize = newSize - Section::flattenedSize() - 4;
+        auto dataSize = newSize - Section::headerSize() - 4;
 
         // Ensure it's 4B aligned
         _data.resize((dataSize / 4) * 4);