LU-9335 pfl: calculate PFL file LOVEA correctly

LU-9335 pfl: calculate PFL file LOVEA correctly

PFL file could contain uninstantiated component, so it could still
keeps the specified -1 stripe count, lov_mds_md_size() should heed
this case, otherwise its LOVEA size could be errneous big.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: Iff741b8ebb1b54d9b27859822735b488e645d85c
Reviewed-on: https://review.whamcloud.com/26611
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>