Adjust g_waitidle() visibility and definition

Explicitly pass the struct thread argument.
Move the function prototype from sys/systm.h to geom/geom.h, we do not
need almost each kernel source to see the prototype, it is now used
only by kern/vfs_mountroot.c outside geom/geom_event.c, where the
function is defined.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D35888