vfs_mountroot: Wait for GEOM idle post root holds

In the case of a root hold related to the initialization of a disk
device, a flurry of GEOM tasting is likely to take place as soon as
the device is initialized and the root hold is released. If we
don't wait for GEOM idle it's easy for vfs_mountroot to "win" the
race and proceed before the root filesystem GEOM is ready.

Reviewed by: imp
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D33592