ovl: Add versioned header for overlay.metacopy xattr

Historically overlay.metacopy was a zero-size xattr, and it's
existence marked a metacopy file. This change adds a versioned header
with a flag field, a length and a digest. The initial use-case of this
will be for validating a fs-verity digest, but the flags field could
also be used later for other new features.

ovl_check_metacopy_xattr() now returns the size of the xattr,
emulating a size of OVL_METACOPY_MIN_SIZE for empty xattrs to
distinguish it from the no-xattr case.

Signed-off-by: Alexander Larsson <alexl@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>