LU-2442 kernel: protect i_dquot with i_lock

LU-2442 kernel: protect i_dquot with i_lock

Remove dqptr_sem (but kept in struct quota_info to keep kernel ABI
unchanged), and the functionality of this lock is implemented by
other locks:
* i_dquot is protected by i_lock, however only this pointer, the
  content of this struct is by dq_data_lock.
* Q_GETFMT is now protected with dqonoff_mutex instead of dqptr_sem.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I6be343fb7e431bb3b0ce68066a36f621ebdd9df5
Reviewed-on: http://review.whamcloud.com/5010
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>