srp/011: Avoid $dev becoming invalid during test

srp/011: Avoid $dev becoming invalid during test

$dev will become invalid when log_out has been done
and fio doesn't run yet. In this case subsequent fio
throws the following error:
-------------------------------------
    From diff -u 011.out 011.out.bad
    Configured SRP target driver
    -Passed

    From 011.full:
    fio: looks like your file system does not support direct=1/buffered=0
    fio: destination does not support O_DIRECT
    run_fio exit code: 1
-------------------------------------
This issue happens randomly.

Try to fix the issue by holding $dev before test.

Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>