LU-6403 quota: fix soft lockup in qmt_adjust_qunit

LU-6403 quota: fix soft lockup in qmt_adjust_qunit

If the user sets the quota limits >= ULLONG_MAX using lfs setquota,
we will set the limits to ULLONG_MAX and stuck in the infinite loop
when trying to increase qunit.

Break the loop when that happens and set qunit to
limit / (2 * slv_cnt) instead.

Signed-off-by: Li Dongyang <dongyang.li@anu.edu.au>
Change-Id: I6fb842c62ad46d8765f6c4c41187cf0dcd543c53
Reviewed-on: http://review.whamcloud.com/14187
Tested-by: Jenkins
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>