L2ARC: Implement per-device feed threads for parallel writes

Transform L2ARC from single global feed thread to per-device threads,
enabling parallel writes to multiple L2ARC devices. Each device runs
its own feed thread independently, improving multi-device throughput.
Previously, a single thread served all devices sequentially; now each
device writes concurrently. Threads are created during device addition
and torn down on removal.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #18093

module/zfs/spa_misc.c

@@ -2668,7 +2668,6 @@ spa_init(spa_mode_t mode)
 	zpool_prop_init();
 	zpool_feature_init();
 	vdev_prop_init();
-	l2arc_start();
 	scan_init();
 	qat_init();
 	spa_import_progress_init();
@@ -2678,8 +2677,6 @@ spa_init(spa_mode_t mode)
 void
 spa_fini(void)
 {
-	l2arc_stop();
-
 	spa_evict_all();
 
 	vdev_file_fini();