zvol: Implement zvol threading as a Property

Currently, zvol threading can be switched through the zvol_request_sync
module parameter system-wide. By making it a zvol property, zvol
threading can be switched per zvol.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #15409

include/sys/fs/zfs.h

@@ -192,6 +192,7 @@ typedef enum {
 	ZFS_PROP_REDACT_SNAPS,
 	ZFS_PROP_SNAPSHOTS_CHANGED,
 	ZFS_PROP_PREFETCH,
+	ZFS_PROP_VOLTHREADING,
 	ZFS_NUM_PROPS
 } zfs_prop_t;
 

include/sys/zvol.h

@@ -50,6 +50,7 @@ extern int zvol_get_stats(objset_t *, nvlist_t *);
 extern boolean_t zvol_is_zvol(const char *);
 extern void zvol_create_cb(objset_t *, void *, cred_t *, dmu_tx_t *);
 extern int zvol_set_volsize(const char *, uint64_t);
+extern int zvol_set_volthreading(const char *, boolean_t);
 extern int zvol_set_common(const char *, zfs_prop_t, zprop_source_t, uint64_t);
 extern zvol_state_handle_t *zvol_suspend(const char *);
 extern int zvol_resume(zvol_state_handle_t *);

include/sys/zvol_impl.h

@@ -58,6 +58,7 @@ typedef struct zvol_state {
 	atomic_t		zv_suspend_ref;	/* refcount for suspend */
 	krwlock_t		zv_suspend_lock;	/* suspend lock */
 	struct zvol_state_os	*zv_zso;	/* private platform state */
+	boolean_t		zv_threading;	/* volthreading property */
 } zvol_state_t;