Fix out-of-bound access when resizing in receive_spill

[tuxoko]

receive_spill will call dbuf_spill_set_blksz, which in turn calls dbuf_new_size when resizing spill block. However, receive_spill holds a raw buf, which can be compressed and less than db_size. This causes an out-of-bound access when dbuf_new_size tries to copy the data using db_size as length.

To fix this, we just disable the copy when coming from receive_spill, since we will assign a new arc_buf later anyway.

Motivation and Context

Description

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair 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.

[tuxoko]

found in #18854 (comment)

[amotin]

I have a subtle feeling this is a dirty workaround. IIRC dbuf layer was never really designed to mix raw and uncompressed buffers, and I wonder if the problem can actually be in db->db.db_size assigned some wrong value, that makes this copy to go out of bounds.