When raw sending in chunks, use the physical payload size

[gamanakis]

Motivation and Context

Closes #12275.

Description

When raw sending and splitting data in chunks dmu_dump_write() will
panic as it tries to dereference a NULL block pointer. In the same case
use the physical payload size instead of the logical one to determine
the number of chunks needed.

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.

[gamanakis]

@pcd1193182 would you mind taking a look? Thanks.

[gamanakis]

After more careful consideration of the current codebase it seems it's not possible to raw send in small chunks. Closing this.