zvol: allow rename of in use ZVOL dataset

While ZFS allow renaming of in use ZVOLs at the DSL level without issues
the ZVOL layer does not correctly update the renamed dataset if the
device node is open (zv->zv_open_count > 0): trying to access the stale
dataset name, for instance during a zfs receive, will cause the
following failure:

VERIFY3(zv->zv_objset->os_dsl_dataset->ds_owner == zv) failed ((null) == ffff8800dbb6fc00)
PANIC at zvol.c:1255:zvol_resume()
Showing stack for process 1390
CPU: 0 PID: 1390 Comm: zfs Tainted: P O 3.16.0-4-amd64 #1 Debian 3.16.51-3
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
0000000000000000 ffffffff8151ea00 ffffffffa0758a80 ffff88028aefba30
ffffffffa0417219 ffff880037179220 ffffffff00000030 ffff88028aefba40
ffff88028aefb9e0 2833594649524556 6f5f767a3e2d767a 6f3e2d7465736a62
Call Trace:
[<0>] ? dump_stack+0x5d/0x78
[<0>] ? spl_panic+0xc9/0x110 [spl]
[<0>] ? mutex_lock+0xe/0x2a
[<0>] ? zfs_refcount_remove_many+0x1ad/0x250 [zfs]
[<0>] ? rrw_exit+0xc8/0x2e0 [zfs]
[<0>] ? mutex_lock+0xe/0x2a
[<0>] ? dmu_objset_from_ds+0x9a/0x250 [zfs]
[<0>] ? dmu_objset_hold_flags+0x71/0xc0 [zfs]
[<0>] ? zvol_resume+0x178/0x280 [zfs]
[<0>] ? zfs_ioc_recv_impl+0x88b/0xf80 [zfs]
[<0>] ? zfs_refcount_remove_many+0x1ad/0x250 [zfs]
[<0>] ? zfs_ioc_recv+0x1c2/0x2a0 [zfs]
[<0>] ? dmu_buf_get_user+0x13/0x20 [zfs]
[<0>] ? alloc_pages_nodemask+0x166/0xb50
[<0>] ? zfsdev_ioctl+0x896/0x9c0 [zfs]
[<0>] ? handle_mm_fault+0x464/0x1140
[<0>] ? do_vfs_ioctl+0x2cf/0x4b0
[<0>] ? do_page_fault+0x177/0x410
[<0>] ? SyS_ioctl+0x81/0xa0
[<0>] ? async_page_fault+0x28/0x30
[<0>] ? system_call_fast_compare_end+0x10/0x15

Reviewed by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6263
Closes #8371