OpenZFS 7143 - dbuf_read() creates unnecessary zio_root() for bonus buf

dbuf_read() creates a zio_root() to track and wait for all the
zio's that may happen as part of this call. However, if the blkptr_t
for this buffer is NULL or a hole, we will not create any more zio's,
so this zio_root() is unnecessary. This is always the case when calling
dbuf_read() on a bonus buffer, because it has no blkptr (it's part of
the containing dnode). For workloads that read a lot of bonus buffers
(e.g. file creation and removal), creating and destroying these
unnecessary zio's can decrease performance by around 3%.

The fix is to only create/destroy the zio_root() in dbuf_read() if
the blkptr is not NULL and not a hole.

Changes sponsored by Intel Corp.

Authored by: Matthew Ahrens <[email protected]>
Reviewed-by:  Alex Zhuravlev <[email protected]>
Ported-by: Brian Behlendorf <[email protected]>
Issue openzfs/openzfs#137
Closes #4803 
Closes #5382 

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