OpenZFS 8025 - dbuf_read() creates unnecessary zio_root() for bonus buf

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

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.

Porting Notes:

• The error handling for when dbuf_read_impl() fails which was originally added in commit 5f6d0b6f5 has been preserved.

OpenZFS-issue: https://www.illumos.org/issues/8025
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/8ec5c7c
Closes #6048