ext4: fix oops in trace_ext4_mb_release_group_pa

Our QA reported an oops in the ext4_mb_release_group_pa tracing,
and Josef Bacik pointed out that it was because we may have a
non-null but uninitialized ac_inode in the allocation context.

I can reproduce it when running xfstests with ext4 tracepoints on, 
on a CONFIG_SLAB_DEBUG kernel.

We call trace_ext4_mb_release_group_pa from 2 places, 
ext4_mb_discard_group_preallocations and 
ext4_mb_discard_lg_preallocations

In both cases we allocate an ac as a container just for tracing (!)
and never fill in the ac_inode.  There's no reason to be assigning,
testing, or printing it as far as I can see, so just remove it from
the tracepoint.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NJosef Bacik <josef@redhat.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

include/trace/events/ext4.h

@@ -432,7 +432,6 @@ TRACE_EVENT(ext4_mb_release_group_pa,
 
 	TP_STRUCT__entry(
 		__field(	dev_t,	dev			)
-		__field(	ino_t,	ino			)
 		__field(	__u64,	pa_pstart		)
 		__field(	__u32,	pa_len			)
 
@@ -440,8 +439,6 @@ TRACE_EVENT(ext4_mb_release_group_pa,
 
 	TP_fast_assign(
 		__entry->dev		= sb->s_dev;
-		__entry->ino		= (ac && ac->ac_inode) ?
-						ac->ac_inode->i_ino : 0;
 		__entry->pa_pstart	= pa->pa_pstart;
 		__entry->pa_len		= pa->pa_len;
 	),