geom(4): make g_newprovider_event() return if G_P_WITHER is set

This fixes a failed assertion in scenario where the provider
disappears, disk_gone() gets called, and at the exact same
time something else closes the device node triggering a retaste.

Reviewed By: mav
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27330