LU-2555 ldiskfs: fix ext4_mb_add_n_trim()

LU-2555 ldiskfs: fix ext4_mb_add_n_trim()

In ext4_mb_add_n_trim(), lg_prealloc_lock should be taken
when changing the lg_prealloc_list.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Icc62d87a682b0ebf70ad12e148a52b864bf1fdb8
Reviewed-on: http://review.whamcloud.com/4961
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>