LU-11768 test: limit at_max to timeout in time

LU-11768 test: limit at_max to timeout in time

In test_6 of sanity-quota, if the AT is enabled, the timeout of
the QUOTA_DQACQ request could be longer than OBD_TIMEOUT*2, which
cause the watchdog to be triggered.

This is a backport to Lustre b2_12 of
Lustre-change: https://review.whamcloud.com/35651
Lustre-commit: d8226b9353dbc1448af8d23c13cae5f21cbe3a86

Change-Id: I7e3a976a004259f5c956fc48f4d8d63c751ee2c0
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36365
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>