Illumos #764: panic in zfs:dbuf_sync_list

Hypothesis about what's going on here.

At some time in the past, something, i.e. dnode_reallocate()
calls one of:
dbuf_rm_spill(dn, tx);

These will do:
dbuf_rm_spill(dnode_t *dn, dmu_tx_t *tx)
dbuf_free_range(dn, DMU_SPILL_BLKID, DMU_SPILL_BLKID, tx)
dbuf_undirty(db, tx)

Currently dbuf_undirty can leave a spill block in dn_dirty_records[],
(it having been put there previously by dbuf_dirty) and free it.
Sometime later, dbuf_sync_list trips over this reference to free'd
(and typically reused) memory.

Also, dbuf_undirty can call dnode_clear_range with a bogus
block ID. It needs to test for DMU_SPILL_BLKID, similar to
how dnode_clear_range is called in dbuf_dirty().

References to Illumos issue and patch:
- https://www.illumos.org/issues/764
- https://github.com/illumos/illumos-gate/commit/3f2366c2bb

Reviewed by: George Wilson <[email protected]>
Reviewed by: [email protected]
Reviewed by: Albert Lee <[email protected]
Approved by: Garrett D'Amore <[email protected]>

Signed-off-by: Brian Behlendorf <[email protected]>
Issue #340

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