LU-12039 tests: fix 'lfs mkdir' in sanity-selinux

LU-12039 tests: fix 'lfs mkdir' in sanity-selinux

sanity-selinux test_2b and test_20c assume that directory created
with 'lfs mkdir' will be on MDT0, but they have to use the '-i 0'
flag to make sure.

Lustre-change: https://review.whamcloud.com/34368
Lustre-commit: e4166f4e9970bbc7ea713d411219a5710c778d61

Test-Parameters: trivial testlist=sanity-selinux clientselinux
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I4c27a93dd8f411cff154c50bb324574179f1a8c8
Reviewed-on: https://review.whamcloud.com/34373
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>