ZIO: Set minimum number of free issue threads to 32

Free issue threads might block waiting for synchronous DDT, BRT or
GANG header reads. So unlike other taskqs using ZTI_SCALE to scale
with number of CPUs, here we also need some amount of threads to
potentially saturate pool reads.  I am not sure we always want the
96 threads we had before ZTI_SCALE introduction at #11966 on small
systems, but lets make it at least 32.

While here, make free taskqs configurable, similar to read and
write ones.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17903

include/os/freebsd/spl/sys/mod.h

@@ -104,6 +104,9 @@
 #define	spa_taskq_write_param_set_args(var) \
     CTLTYPE_STRING, NULL, 0, spa_taskq_write_param, "A"
 
+#define	spa_taskq_free_param_set_args(var) \
+    CTLTYPE_STRING, NULL, 0, spa_taskq_free_param, "A"
+
 #define	fletcher_4_param_set_args(var) \
     CTLTYPE_STRING, NULL, 0, fletcher_4_param, "A"