Some additional send stream validity checking

[nedbass]

Description

Check in the DMU whether an object record in a send stream being
received contains an unsupported dnode slot count, and return an
error if it does. Failure to catch an unsupported dnode slot count
would result in a panic when the SPA attempts to increment the
reference count for the large_dnode feature and the pool has the
feature disabled. This is not normally an issue for a well-formed
send stream which would have the DMU_BACKUP_FEATURE_LARGE_DNODE flag
set if it contains large dnodes, so it will be rejected as
unsupported if the required feature is disabled. This change adds a
missing object record field validation.

Add missing stream feature flag checks in
dmu_recv_resume_begin_check().

Consolidate repetitive comment blocks in dmu_recv_begin_check().

Update zstreamdump to print the dnode slot count (dn_slots) for an
object record when running in verbose mode.

Motivation and Context

Kernel-side hardening of backup stream receive code.

How Has This Been Tested?

I generated a backup stream that contained large dnodes but was missing the DMU_BACKUP_FEATURE_LARGE_DNODE flag. I then tried to receive this stream in a pool
created with -o version=28. Without this patch, the node panicked due to a failed
VERIFY in the feature reference count code. With this patch, the stream is safely rejected
by the kernel.

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.

[ahrens]

This change is fine, but I think the kernel also needs to check this in dmu_recv_resume_begin_check(), where it checks the other featureflags.

[nedbass]

@ahrens I added checks for both large block and large dnode feature flags.

[ahrens]

@nedbass Thanks. We have the check for LARGE_BLOCKS there in Delphix but looks like that fix was made as part of a larger restructuring that @pcd1193182 is still working on upstreaming.