linux/zvol_os: fix zvol queue limits initialization

zvol queue limits initialization depends on `zv_volblocksize`, but it is
initialized later, leading to several limits being initialized with
incorrect values, including `max_discard_*` limits. This also causes
`blkdiscard` command to consistently fail, as `blk_ioctl_discard` reads
`bdev_max_discard_sectors()` limits as 0, leading to failure. The fix is
straightforward: initialize `zv->zv_volblocksize` early, before setting
the queue limits. This PR should fix `zvol/zvol_misc/zvol_misc_trim`
failure on recent PRs, as the test case issues `blkdiscard` for a zvol.
Additionally, `zvol_misc_trim` was recently enabled in `6c7d41a`,
which is why the issue wasn't identified earlier.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Ameer Hamza <[email protected]>
Closes openzfs#16454

module/os/linux/zfs/zvol_os.c

@@ -1362,7 +1362,7 @@ zvol_alloc_blk_mq(zvol_state_t *zv, zvol_queue_limits_t *limits)
  * request queue and generic disk structures for the block device.
  */
 static zvol_state_t *
-zvol_alloc(dev_t dev, const char *name)
+zvol_alloc(dev_t dev, const char *name, uint64_t volblocksize)
 {
 	zvol_state_t *zv;
 	struct zvol_state_os *zso;
@@ -1382,6 +1382,7 @@ zvol_alloc(dev_t dev, const char *name)
 	zso = kmem_zalloc(sizeof (struct zvol_state_os), KM_SLEEP);
 	zv->zv_zso = zso;
 	zv->zv_volmode = volmode;
+	zv->zv_volblocksize = volblocksize;
 
 	list_link_init(&zv->zv_next);
 	mutex_init(&zv->zv_state_lock, NULL, MUTEX_DEFAULT, NULL);
@@ -1671,7 +1672,8 @@ zvol_os_create_minor(const char *name)
 	if (error)
 		goto out_dmu_objset_disown;
 
-	zv = zvol_alloc(MKDEV(zvol_major, minor), name);
+	zv = zvol_alloc(MKDEV(zvol_major, minor), name,
+	    doi->doi_data_block_size);
 	if (zv == NULL) {
 		error = SET_ERROR(EAGAIN);
 		goto out_dmu_objset_disown;
@@ -1681,7 +1683,6 @@ zvol_os_create_minor(const char *name)
 	if (dmu_objset_is_snapshot(os))
 		zv->zv_flags |= ZVOL_RDONLY;
 
-	zv->zv_volblocksize = doi->doi_data_block_size;
 	zv->zv_volsize = volsize;
 	zv->zv_objset = os;