Use a dedicated taskq for vdev_file

Originally, vdev_file used system_taskq. This would cause a deadlock,
especially on system with few CPUs. The reason is that the prefetcher
threads, which are on system_taskq, will sometimes be blocked waiting
for I/O to finish. If the prefetcher threads consume all the tasks in
system_taskq, the I/O cannot be served and thus results in a deadlock.

We fix this by creating a dedicated vdev_file_taskq for vdev_file I/O.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2270

module/zfs/spa_misc.c

@@ -34,6 +34,7 @@
 #include <sys/zap.h>
 #include <sys/zil.h>
 #include <sys/vdev_impl.h>
+#include <sys/vdev_file.h>
 #include <sys/metaslab.h>
 #include <sys/uberblock_impl.h>
 #include <sys/txg.h>
@@ -1660,6 +1661,7 @@ spa_init(int mode)
 	dmu_init();
 	zil_init();
 	vdev_cache_stat_init();
+	vdev_file_init();
 	zfs_prop_init();
 	zpool_prop_init();
 	zpool_feature_init();
@@ -1674,6 +1676,7 @@ spa_fini(void)
 
 	spa_evict_all();
 
+	vdev_file_fini();
 	vdev_cache_stat_fini();
 	zil_fini();
 	dmu_fini();