geom: fix initialization order

There's a race between the initialization of devsoftc.mtx (by devinit)
and the creation of the geom worker thread g_run_events, which calls
devctl_queue_data_f. Both of those are initialized at SI_SUB_DRIVERS
and SI_ORDER_FIRST, which means the geom worked thread can be created
before the mutex has been initialized, leading to the panic below:

 wpanic: mtx_lock() of spin mutex (null) @ /usr/home/osstest/build.135317.build-amd64-freebsd/freebsd/sys/kern/subr_bus.c:620
 cpuid = 3
 time = 1
 KDB: stack backtrace:
 db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe003b968710
 vpanic() at vpanic+0x19d/frame 0xfffffe003b968760
 panic() at panic+0x43/frame 0xfffffe003b9687c0
 __mtx_lock_flags() at __mtx_lock_flags+0x145/frame 0xfffffe003b968810
 devctl_queue_data_f() at devctl_queue_data_f+0x6a/frame 0xfffffe003b968840
 g_dev_taste() at g_dev_taste+0x463/frame 0xfffffe003b968a00
 g_load_class() at g_load_class+0x1bc/frame 0xfffffe003b968a30
 g_run_events() at g_run_events+0x197/frame 0xfffffe003b968a70
 fork_exit() at fork_exit+0x84/frame 0xfffffe003b968ab0
 fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe003b968ab0
 --- trap 0, rip = 0, rsp = 0, rbp = 0 ---
 KDB: enter: panic
 [ thread pid 13 tid 100029 ]
 Stopped at      kdb_enter+0x3b: movq    $0,kdb_why

Fix this by initializing geom at SI_ORDER_SECOND instead of
SI_ORDER_FIRST.

Sponsored by:		Citrix Systems R&D
Reviewed by:		kevans, markj
Differential revision:	https://reviews.freebsd.org/D20148

sys/geom/geom.h

@@ -415,7 +415,7 @@ g_free(void *ptr)
 	static moduledata_t name##_mod = {			\
 		#name, g_modevent, &class			\
 	};							\
-	DECLARE_MODULE(name, name##_mod, SI_SUB_DRIVERS, SI_ORDER_FIRST);
+	DECLARE_MODULE(name, name##_mod, SI_SUB_DRIVERS, SI_ORDER_SECOND);
 
 int g_is_geom_thread(struct thread *td);