Remove refcount from spa_config_*()

The only reason for spa_config_*() to use refcount instead of simple
non-atomic (thanks to scl_lock) variable for scl_count is tracking,
hard disabled for the last 8 years.  Switch to simple int scl_count
reduces the lock hold time by avoiding atomic, plus makes structure
fit into single cache line, reducing the locks contention.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12287

include/sys/spa_impl.h

@@ -146,9 +146,9 @@ typedef struct spa_config_lock {
 	kmutex_t	scl_lock;
 	kthread_t	*scl_writer;
 	int		scl_write_wanted;
+	int		scl_count;
 	kcondvar_t	scl_cv;
-	zfs_refcount_t	scl_count;
-} spa_config_lock_t;
+} ____cacheline_aligned spa_config_lock_t;
 
 typedef struct spa_config_dirent {
 	list_node_t	scd_link;