Factor out some dbuf subroutines and add state change tracing

Create dedicated dbuf_read_hole and dbuf_read_bonus.
Additionally, add a dtrace probe to allow state change tracing.

Reviewed-by: Matt Ahrens <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Will Andrews <[email protected]>
Reviewed by: Brad Lewis <[email protected]>
Authored-by: Will Andrews <[email protected]>
Signed-off-by: Matt Macy <[email protected]>
Closes #9923 

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