ext4: do not reference pa_inode from group_pa

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>

fs/ext4/mballoc.c

@@ -3671,7 +3671,7 @@ ext4_mb_release_group_pa(struct ext4_buddy *e4b,
 	ext4_group_t group;
 	ext4_grpblk_t bit;
 
-	trace_ext4_mb_release_group_pa(pa);
+	trace_ext4_mb_release_group_pa(sb, pa);
 	BUG_ON(pa->pa_deleted == 0);
 	ext4_get_group_no_and_offset(sb, pa->pa_pstart, &group, &bit);
 	BUG_ON(group != e4b->bd_group && pa->pa_len != 0);

include/trace/events/ext4.h

@@ -573,9 +573,9 @@ TRACE_EVENT(ext4_mb_release_inode_pa,
 );
 
 TRACE_EVENT(ext4_mb_release_group_pa,
-	TP_PROTO(struct ext4_prealloc_space *pa),
+	TP_PROTO(struct super_block *sb, struct ext4_prealloc_space *pa),
 
-	TP_ARGS(pa),
+	TP_ARGS(sb, pa),
 
 	TP_STRUCT__entry(
 		__field(	dev_t,	dev			)
@@ -585,7 +585,7 @@ TRACE_EVENT(ext4_mb_release_group_pa,
 	),
 
 	TP_fast_assign(
-		__entry->dev		= pa->pa_inode->i_sb->s_dev;
+		__entry->dev		= sb->s_dev;
 		__entry->pa_pstart	= pa->pa_pstart;
 		__entry->pa_len		= pa->pa_len;
 	),