Fix compiler warnings

Building with a newer version of gcc exposed some cases where
values should be explicitly cast.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Add --random/--seed options

Allow a set of randomly sized xattrs to be set on each file.  When
the verify option is set the expected size of these xattrs is checked
along with a trivial pattern.

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