Reduce zdb output when pool contains checkpoint

[behlendorf]

Description

When running zdb without additional arguments against a pool containing
a checkpoint the entire checkpoint spacemap should not be dumped. Make
this behavior conditional upon passing the -mmmm option as described in
the zdb(8) man page.

 -mmmm   Display every spacemap record.

Motivation and Context

Keep the size of the ZTS log files manageable so they're relatively easy to read.

How Has This Been Tested?

Manual testing to verify the log messages are only printed when passed -mmmm.

Reduced the size of the zpool_checkpoint logs from 25M to 748K.

$ du -h  /var/tmp/test_results/20180710T134551/pool_checkpoint | tail -1
25M     /var/tmp/test_results/20180710T134551/pool_checkpoint

$ du -h  /var/tmp/test_results/20180710T143330/pool_checkpoint | tail -1
748K    /var/tmp/test_results/20180710T143330/pool_checkpoint

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.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[sdimitro]

LGTM.
Thanks for fixing this Brian!

[codecov]

Codecov Report

Merging #7702 into master will decrease coverage by 0.05%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #7702      +/-   ##
==========================================
- Coverage   78.11%   78.06%   -0.06%     
==========================================
  Files         368      368              
  Lines      111830   111831       +1     
==========================================
- Hits        87354    87297      -57     
- Misses      24476    24534      +58

Flag	Coverage Δ
#kernel	78.69% <ø> (-0.04%)	⬇️
#user	67.22% <50%> (+0.19%)	⬆️

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