Check if the provided flags value from the vsock address data structure
includes the supported flags in the corresponding kernel version.

The first byte of the "svm_zero" field is used as "svm_flags", so add
the flags check instead.

Changelog

v3 -> v4

* New patch in v4.

Signed-off-by: Andra Paraschiv <andraprs@amazon.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>