MFV/ZoL: Fix PANIC: metaslab_free_dva(): bad DVA X:Y:Z

MFV/ZoL:    Fix PANIC: metaslab_free_dva(): bad DVA X:Y:Z

commit 81edd3e83409218879e7af293daa86b0c40eb015
Author: Peng <peng.hse@xtaotech.com>
Date:   Wed Jun 8 15:22:07 2016 +0800

    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