Remove mg_allocators (#17192)

Previous code allowed each metaslab group to have different number
of allocators.  But in practice it worked only for embedded SLOGs,
relying on a number of conditions and creating a significant mine
field if any of those change.  I just stepped on one myself.

This change makes all groups to have spa_alloc_count allocators.
It may cost us extra 192 bytes of memory per normal top-level vdev
on large systems, but I find it a small price for cleaner and more
reliable code.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Fixes #17188
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>

include/sys/metaslab.h

@@ -118,7 +118,7 @@ uint64_t metaslab_class_get_deferred(metaslab_class_t *);
 void metaslab_space_update(vdev_t *, metaslab_class_t *,
     int64_t, int64_t, int64_t);
 
-metaslab_group_t *metaslab_group_create(metaslab_class_t *, vdev_t *, int);
+metaslab_group_t *metaslab_group_create(metaslab_class_t *, vdev_t *);
 void metaslab_group_destroy(metaslab_group_t *);
 void metaslab_group_activate(metaslab_group_t *);
 void metaslab_group_passivate(metaslab_group_t *);

include/sys/metaslab_impl.h

@@ -269,7 +269,6 @@ struct metaslab_group {
 	kmutex_t		mg_ms_disabled_lock;
 	kcondvar_t		mg_ms_disabled_cv;
 
-	int			mg_allocators;
 	metaslab_group_allocator_t	mg_allocator[];
 };