Fix PANIC: metaslab_free_dva(): bad DVA X:Y:Z

Fix PANIC: metaslab_free_dva(): bad DVA X:Y:Z

The following scenario can result in garbage in the dn_spill field.
The db->db_blkptr must be set to NULL when DNODE_FLAG_SPILL_BLKPTR
is clear to ensure the dn_spill field is cleared.

Current txg = A.
* A new spill buffer is created. Its dbuf is initialized with
  db_blkptr = NULL and it's dirtied.

Current txg = B.
* The spill buffer is modified. It's marked as dirty in this txg.
* Additional changes make the spill buffer unnecessary because the
  xattr fits into the bonus buffer, so it's removed. The dbuf is
  undirtied in this txg, but it's still referenced and cannot be
  destroyed.

Current txg = C.
* Starts syncing of txg A
* dbuf_sync_leaf() is called for the spill buffer. Since db_blkptr
  is NULL, dbuf_check_blkptr() is called.
* The dbuf starts being written and it reaches the ready state
  (not done yet).
* A new change makes the spill buffer necessary again.
  sa_build_layouts() ends up calling dbuf_find() to locate the
  dbuf.  It finds the old dbuf because it has not been destroyed yet
  (it will be destroyed when the previous write is done and there
  are no more references). The old dbuf has db_blkptr != NULL.
* txg A write is complete and the dbuf released. However it's still
  referenced, so it's not destroyed.

Current txg = D.
* Starts syncing of txg B
* dbuf_sync_leaf() is called for the bonus buffer. Its contents are
  directly copied into the dnode, overwriting the blkptr area because,
  in txg B, the bonus buffer was big enough to hold the entire xattr.
* At this point, the db_blkptr of the spill buffer used in txg C
  gets corrupted.

Signed-off-by: Peng <peng.hse@xtaotech.com>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3937