Slightly improve dnode hash

[amotin]

As I understand just for being less predictable dnode hash includes 8 bits of objset pointer, starting at 6. But since objset_t is more than 1KB in size, its allocations are likely aligned to 2KB, that means 11 lower bits provide no entropy. Just take the 8 bits starting from 11.

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)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil 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.

[amotin]

I wonder if cityhash4() could be better (faster or cleaner) here.

[Harry-Chen]

I wonder if cityhash4() could be better (faster or cleaner) here.

I did some quick test: https://godbolt.org/z/o6j4dPEvK. On most platforms, with -O2 level optimization, cityhash64 generates fewer instructions than CRC64 and eliminates all memory access.

P.S. cityhash64 is excerpted from https://github.com/google/cityhash/blob/master/src/city.cc