witness: change blessed table to a linker set

[kvinupriya_gmail.com]

Summary

This allows for modules and other parts of the kernel to potentially
contribute to the list.

The WITNESS_BLESSED macro takes two arguments, both are names of the
locks that witness should not complain about any order problems
between them.

Caveat: modules using the WITNESS_BLESSED macro and built with
WITNESS enabled will not load on non-WITNESS kernels.

• Add blessed witness entry for tarfs and ufs

Sponsored by: Juniper Networks, Inc.