xattrtest: allow verify with -R and other improvements

xattrtest: allow verify with -R and other improvements

- Use a fixed buffer of random bytes when random xattr values are in
  effect.  This eliminates the potential performance bottleneck of
  reading from /dev/urandom for each file. This also allows us to
  verify xattrs in random value mode.

- Show the rate of operations per second in addition to elapsed time
  for each phase of the test. This may be useful for benchmarking.

- Set default xattr size to 6 so that verify doesn't fail if user
  doesn't specify a size. We need at least six bytes to store the
  leading "size=X" string that is used for verification.

- Allow user to execute just one phase of the test. Acceptable
  values for -o and their meanings are:

   1 - run the create phase
   2 - run the setxattr phase
   3 - run the getxattr phase
   4 - run the unlink phase

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>