geom(4) mirror: Do not panic on gmirror(8) insert, resize

Geom_mirror initialization occurs in spurts and the present of a
non-destroyed g_mirror softc does not always indicate that the geom has
launched (i.e., has an sc_provider).

Some gmirror(8) commands (via g_mirror_ctl) depend on a g_mirror's
sc_provider (insert and resize). For those commands, g_mirror_ctl is
modified to sleep-poll in an interruptible way until the target geom is
either launched or destroyed.

Reviewed by: markj
Tested by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D24780