CVE-2021-44142: libadouble: harden ad_unpack_xattrs()

CVE-2021-44142: libadouble: harden ad_unpack_xattrs()

This ensures ad_unpack_xattrs() is only called for an ad_type of ADOUBLE_RSRC,
which is used for parsing ._ AppleDouble sidecar files, and the buffer
ad->ad_data is AD_XATTR_MAX_HDR_SIZE bytes large which is a prerequisite for all
buffer out-of-bounds access checks in ad_unpack_xattrs().

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14914

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