pa_inode in group_pa is set NULL in ext4_mb_new_group_pa, so
pa_inode should be not referenced.
Reported-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NYongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>