engines/hdfs: swap fio_hdfsio_init and fio_hdfsio_io_u_init

engines/hdfs: swap fio_hdfsio_init and fio_hdfsio_io_u_init

Commit 08dc3bd50 initialized the io engine before the io_u buffers. The
unintended consequence for the libhdfs io engine was that the HDFS
connection was not established before attempting to open HDFS file
handles. This caused a NPE in the java layer on start-up preventing the
libhdfs io engine from being able to be used.

Signed-off-by: Albert Chang <albert.chang@hitachivantara.com>