Remove duplicate dnode.h include

[behlendorf]

Motivation and Context

Issue #10816

Description

The zfs/sa.c source file accidentally includes sys/dnode.h twice.
Remove the second occurance.

How Has This Been Tested?

Pending CI results.

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]

Codecov Report

Merging #10819 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10819      +/-   ##
==========================================
+ Coverage   79.47%   79.64%   +0.16%     
==========================================
  Files         395      395              
  Lines      125066   125066              
==========================================
+ Hits        99401    99612     +211     
+ Misses      25665    25454     -211     

Flag	Coverage Δ
#kernel	80.37% <ø> (+0.06%)	⬆️
#user	65.41% <ø> (+0.25%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
module/zfs/sa.c	84.19% <ø> (+0.10%)	⬆️
module/zcommon/zfs_fletcher.c	68.09% <0.00%> (-7.57%)	⬇️
module/lua/lmem.c	83.33% <0.00%> (-4.17%)	⬇️
module/zcommon/zfs_fletcher_superscalar.c	97.05% <0.00%> (-2.95%)	⬇️
lib/libzpool/kernel.c	64.69% <0.00%> (-2.28%)	⬇️
module/icp/api/kcf_mac.c	36.57% <0.00%> (-1.72%)	⬇️
module/zcommon/zfs_fletcher_superscalar4.c	98.43% <0.00%> (-1.57%)	⬇️
module/zcommon/zfs_fletcher_sse.c	98.66% <0.00%> (-1.34%)	⬇️
lib/libzfs/libzfs_changelist.c	85.01% <0.00%> (-1.13%)	⬇️
module/os/linux/zfs/zfs_file_os.c	84.15% <0.00%> (-1.00%)	⬇️
... and 54 more

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