Enable zero-elimination if dedup is activated

[allanjude]

Motivation and Context

Optimize dedup's handling of blocks of all-zeros.

Description

Creating high-refcount dedup entries for checksums of each record size is inefficient when the record could be recorded as a hole instead.

Normally, zero-elimination is disabled if compression is not enabled, but in the dedup case, it is expected that the write will be skipped anyway, we are just optimizing the way the write is skipped.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

@allanjude There seem to be some edge cases:

  [ 7192.631153] ZTS run /usr/share/zfs/zfs-tests/tests/functional/rsend/send-wR_encrypted_zvol
...
  [ 7195.123099] VERIFY3U(off + size, <=, abd->abd_size) failed (16384 <= 1024)
  [ 7195.125098] PANIC at abd.c:772:abd_iterate_func()
  [ 7195.126829] Showing stack for process 614070
  [ 7195.128264] CPU: 1 PID: 614070 Comm: z_wr_iss Tainted: P           OE      6.1.0-34-amd64 #1  Debian 6.1.135-1
  [ 7195.132048] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
  [ 7195.135981] Call Trace:
  [ 7195.138731]  <TASK>
  [ 7195.141046]  dump_stack_lvl+0x44/0x5c
  [ 7195.143620]  spl_panic+0xf0/0x108 [spl]
  [ 7195.146237]  ? vdev_disk_io_rw+0xb9/0x1e0 [zfs]
  [ 7195.158061]  abd_iterate_func+0x163/0x190 [zfs]
  [ 7195.161238]  ? abd_get_from_buf_impl+0x70/0x70 [zfs]
  [ 7195.162996]  zio_write_compress+0x9c9/0x11b0 [zfs]
  [ 7195.165429]  ? srso_alias_return_thunk+0x5/0x7f
  [ 7195.167039]  zio_execute+0xea/0x200 [zfs]
  [ 7195.168533]  taskq_thread+0x27d/0x5a0 [spl]
  [ 7195.169926]  ? wake_up_q+0x90/0x90
  [ 7195.171443]  ? zio_execute_stack_check.constprop.0+0x10/0x10 [zfs]
  [ 7195.174046]  ? taskq_lowest_id+0xc0/0xc0 [spl]
  [ 7195.175578]  kthread+0xda/0x100
  [ 7195.176777]  ? kthread_complete_and_exit+0x20/0x20
  [ 7195.178362]  ret_from_fork+0x22/0x30
  [ 7195.179891]  </TASK>