LU-13417 test: mkdir_on_mdt0() in more tests

LU-13417 test: mkdir_on_mdt0() in more tests

Replace mkdir with mkdir_on_mdt0() in several tests.

Update recovery-small test_110k() in case there are opened files on
MDT1 which would cause umount stall.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Iebc32568b7fc146b658f47c5f5053fd3db24432f
Reviewed-on: https://review.whamcloud.com/44315
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>