adouble: use runtime size when writing out AppleDouble header

adouble: use runtime size when writing out AppleDouble header

We're calling SMB_VFS_NEXT_PWRITE() to write out the AppleDouble header
data. Until now this header always had a fixed sizeof AD_DATASZ_DOT_UND bytes,
but a subsequent commit will change that. Use the correct size which is the
offset of the begining of the resource fork data.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>