ntfs: fix VCN overflow in ntfs_mapping_pairs_decompress()

ntfs: fix VCN overflow in ntfs_mapping_pairs_decompress()

In ntfs_mapping_pairs_decompress(), lowest_vcn is read from
on-disk metadata and used as the initial vcn without validation.
A malformed value can introduce an invalid (e.g. negative) vcn,
corrupting the runlist from the start.

Additionally, the accumulation
    vcn += deltaxcn

does not check for s64 overflow. A crafted mapping pairs array
can wrap vcn to a negative value, breaking the monotonically-
increasing invariant relied upon by ntfs_rl_vcn_to_lcn() and
related helpers.

Fix this by validating lowest_vcn and using check_add_overflow()
for vcn accumulation.

Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>