LU-4310 kernel: missed unlock in quota patch

LU-4310 kernel: missed unlock in quota patch

The quota-replace-dqptr-sem.patch introduced a regression which
could miss unlock inode->i_lock in __dquot_alloc_space().

This patch is back-ported from the following patch:
Lustre-commit: 1fa094b5977820127ccae4ea7c13904c9716c6fb
Lustre-change: http://review.whamcloud.com/8400

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I2353a8f60ea0b4d54332eaafcb2397f6c0ed28e6
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: http://review.whamcloud.com/8442
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>