vfs_mountroot: Skip 'Root mount waiting' < 1 s

While the message is technically correct, it's not particularly
helpful in the case where we're only waiting a few ms; this case
occurs frequently on EC2 arm64 instances with CAM initialization
racing to release its root hold before vfs_mountroot reaches this
point. Only print the message if we end up waiting for more than
one second.

Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D33591

(cherry picked from commit e6db5eb9ec7ba9edfc564fd5ade5fb40df9acebb)