remove unneeded member drc_err of dmu_recv_cookie_t

[ahrens]

Motivation and Context

code cleanup

Description

The member drc_err of dmu_recv_cookie_t is used only locally in
receive_read, so we can replace it with a local variable.

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)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux 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.

[codecov-io]

Codecov Report

Merging #10319 into master will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #10319      +/-   ##
==========================================
+ Coverage   79.48%   79.64%   +0.15%     
==========================================
  Files         389      389              
  Lines      123128   123128              
==========================================
+ Hits        97869    98061     +192     
+ Misses      25259    25067     -192     

Flag	Coverage Δ
#kernel	80.04% <100.00%> (+0.07%)	⬆️
#user	66.12% <0.00%> (+0.57%)	⬆️

Impacted Files	Coverage Δ
module/zfs/dmu_recv.c	76.53% <100.00%> (+0.14%)	⬆️
module/zfs/vdev_indirect.c	74.00% <0.00%> (-11.00%)	⬇️
module/zfs/dsl_synctask.c	92.40% <0.00%> (-2.54%)	⬇️
module/os/linux/spl/spl-kmem-cache.c	83.72% <0.00%> (-1.27%)	⬇️
module/zfs/vdev_queue.c	95.10% <0.00%> (-0.92%)	⬇️
module/os/linux/zfs/vdev_disk.c	84.36% <0.00%> (-0.73%)	⬇️
module/zfs/zfs_fm.c	84.39% <0.00%> (-0.58%)	⬇️
module/zfs/vdev_indirect_mapping.c	98.55% <0.00%> (-0.49%)	⬇️
module/os/linux/zfs/zfs_vfsops.c	75.83% <0.00%> (-0.12%)	⬇️
module/zfs/metaslab.c	95.74% <0.00%> (-0.10%)	⬇️
... and 45 more

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