ZIO: ZIO_STAGE_DDT_WRITE is a blocking stage

[amotin]

ddt_lookup() in zio_ddt_write() might require synchronous DDT ZAP read. Running it from interrupt taskq might lead to deadlock. Inclusion of ZIO_STAGE_DDT_WRITE into ZIO_BLOCKING_STAGES should hopefully fix that, even though I am not sure how I got there.

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[behlendorf]

Sure enough, ZIO_STAGE_DDT_WRITE might require a synchronous read and that could deadlock under the right circumstances. It makes me wonder if there are any other pipeline stages with the same subtle issue. It didn't see anything obvious with a quick spot check. It'd be nice to somehow add some assertions to catch this kind of thing.