vfs_domount_update(): postpone setting MNT_UNION until VFS_MOUNT() is done

The file system that handles updating the mount point might do lookups
during the update, in which case it could find the flag MNT_UNION set on
the mp while mount point is still not updated. In particular, the
rootvp->v_mount->mnt_vnodecovered is not yet set.

Delay setting MNT_UNION until the mount is performed.

PR: 265311
Reported by: Robert Morris <rtm@lcs.mit.edu>
Reviewed by: mckusick, olce
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D45208