Verify the return value for warn_unused_result functions

Verify the return value for warn_unused_result functions

Under Linux the following functions are flagged with the
attribute warn_unused_result, this triggers a warning when
ever they are used without checking the return value.

To handle this case we check the result VERIFY().  It's
better to detect this immediately on failure rather than
segfault farther down in the function.

  ../../cmd/ztest/ztest.c:6033:2: warning:
  ignoring return value of 'asprintf', declared with
  attribute warn_unused_result [-Wunused-result]
  ../../cmd/ztest/ztest.c:739:3: warning:
  ignoring return value of 'realpath', declared with
  attribute warn_unused_result [-Wunused-result]

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