LU-1688 test: fix the problem in test_58 of recovery_small

LU-1688 test: fix the problem in test_58 of recovery_small

in test_58 of recovery_small, the 'df' command could trigger
the eviction caused by the above 'drop_bl_callback', then
the fail of 'df' could be not thought as the fail of th test,

Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Change-Id: I226a3d4772d033182ded28caacbecdc44c7af8c4
Reviewed-on: http://review.whamcloud.com/3506
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jay J Lan <jay.j.lan@nasa.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>