Prefetching of dnodes in dbuf_read() can cause significant mutex
contention for some workloads and isn't very helpful. This is
because we already get 32 dnodes for each block read, and when
iterating over a directory we prefetch the dnodes in the directory.
Disable this prefetching to prevent the lock contention.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Submitted-by: Adam Moss <c@yotes.com> Submitted-by: Matthew Ahrens <mahrens@delphix.com> Signed-off-by: Adam Moss <c@yotes.com>
Closes #10877
Closes #10953