x86, bts: detect size of DS fields, fix

Impact: build fix

One usage site was missed in the sizeof_field -> sizeof_ptr_field
rename.

Cc: Markus Metzger <markus.t.metzger@intel.com>
LKML-Reference: <20090313104218.A30096@sedona.ch.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>