OpenZFS 9577 - remove zfs_dbuf_evict_key tsd

The zfs_dbuf_evict_key TSD (thread-specific data) is not necessary -
we can instead pass a flag down in a few places to prevent recursive
dbuf eviction. Making this change has 3 benefits:

1. The code semantics are easier to understand.
2. On Linux, performance is improved, because creating/removing
   TSD values (by setting to NULL vs non-NULL) is expensive, and
   we do it very often.
3. According to Nexenta, the current semantics can cause a
   deadlock when concurrently calling dmu_objset_evict_dbufs()
   (which is rare today, but they are working on a "parallel
   unmount" change that triggers this more easily):

Porting Notes:
* Minor conflict with OpenZFS 9337 which has not yet been ported.

Authored by: Matthew Ahrens <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Serapheim Dimitropoulos <[email protected]>
Reviewed by: Brad Lewis <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Ported-by: Brian Behlendorf <[email protected]>

OpenZFS-issue: https://illumos.org/issues/9577
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/645
External-issue: DLPX-58547
Closes #7602

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