LU-4573 tests: check all MDTs for open files

sanityn.sh test_76 verifying mdt.*.exports.*.open_files content
did not check all of the MDTs. It was broken by commit 370de927fc5,
which created all directories striped across all MDTs by default.

There is actually no need to special-case striped or remote
directories in this test, and it should be possible to just check
for open files on all MDTs, and this also simplifies the test.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I2b7b434052946c8d84bffef1fc7358cc29b00b75
Reviewed-on: http://review.whamcloud.com/10107
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>