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>