fs/ntfs: Implement attribute verification

fs/ntfs: Implement attribute verification

It was possible to read OOB when an attribute had a size that exceeded
the allocated buffer. This resolves that by making sure all attributes
that get read are fully in the allocated space by implementing
a function to validate them.

Defining the offsets in include/grub/ntfs.h but they are only used in
the validation function and not across the rest of the NTFS code.

Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>