ceph: fix pending vmtruncate race

The locking order for pending vmtruncate is wrong, it can lead to
following race:

        write                  wmtruncate work
------------------------    ----------------------
lock i_mutex
check i_truncate_pending   check i_truncate_pending
truncate_inode_pages()     lock i_mutex (blocked)
copy data to page cache
unlock i_mutex
                           truncate_inode_pages()

The fix is take i_mutex before calling __ceph_do_pending_vmtruncate()

Fixes: http://tracker.ceph.com/issues/5453


Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>