Need a long hold in zpl_mount_impl

In zpl_mount_impl, there is:
    dmu_objset_hold	; returns with pool & ds held
    dsl_pool_rele

    sget

    dsl_dataset_rele

As spelled out in the "DSL Pool Configuration Lock" in dsl_pool.c,
this requires a long hold.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Paul Zuchowski <[email protected]>
Signed-off-by: John Poduska <[email protected]>
Closes openzfs#10936

module/os/linux/zfs/zpl_super.c

@@ -274,8 +274,12 @@ zpl_mount_impl(struct file_system_type *fs_type, int flags, zfs_mnt_t *zm)
 	 * a txg sync.  If the dsl_pool lock is held over sget()
 	 * this can prevent the pool sync and cause a deadlock.
 	 */
+	dsl_dataset_long_hold(dmu_objset_ds(os), FTAG);
 	dsl_pool_rele(dmu_objset_pool(os), FTAG);
+
 	s = sget(fs_type, zpl_test_super, set_anon_super, flags, os);
+
+	dsl_dataset_long_rele(dmu_objset_ds(os), FTAG);
 	dsl_dataset_rele(dmu_objset_ds(os), FTAG);
 
 	if (IS_ERR(s))