zdb: enhancement - Display SA xattrs.

If the znode has SA xattrs, display them following the other
standard attributes. The format used is similar to that used
when listing the contents of a ZAP. It is as follows:

$ zdb -vvv <pool>/<dataset> <object>
...
SA xattrs: <size> bytes, <number> entries

		<name1> = <value1>
		<name2> = <value2>
		...

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1581