MdePkg: Follow PI spec to update ExtendedSize in EFI_FFS_FILE_HEADER2

MdePkg: Follow PI spec to update ExtendedSize in EFI_FFS_FILE_HEADER2

for FFS data above 16 bytes alignment requirement.

PI spec requires FFS header to be at 8 bytes alignment to FV header.
And, FFS data alignment requires the beginning of the file data must
be aligned on a particular boundary, such as 1, 16, 128 bytes or above.
If FFS data alignment requires to be above 16 bytes, and FFS header
must be at 8 byte alignment, so FFS header size must be multiple of 8.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18263 6f19259b-4bc3-4df7-8a09-765794883524