xfs: Fix xfs_attr_leafblock definition

struct xfs_attr_leafblock contains 'entries' array which is declared
with size 1 altough it can in fact contain much more entries. Since this
array is followed by further struct members, gcc (at least in version
4.8.3) thinks that the array has the fixed size of 1 element and thus
may optimize away all accesses beyond the end of array resulting in
non-working code. This problem was only observed with userspace code in
xfsprogs, however it's better to be safe in kernel as well and have
matching kernel and xfsprogs definitions.

cc: <stable@vger.kernel.org>
Signed-off-by: Jan Kara <jack@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_da_format.h

@@ -680,8 +680,15 @@ typedef struct xfs_attr_leaf_name_remote {
 typedef struct xfs_attr_leafblock {
 	xfs_attr_leaf_hdr_t	hdr;	/* constant-structure header block */
 	xfs_attr_leaf_entry_t	entries[1];	/* sorted on key, not name */
-	xfs_attr_leaf_name_local_t namelist;	/* grows from bottom of buf */
-	xfs_attr_leaf_name_remote_t valuelist;	/* grows from bottom of buf */
+	/*
+	 * The rest of the block contains the following structures after the
+	 * leaf entries, growing from the bottom up. The variables are never
+	 * referenced and definining them can actually make gcc optimize away
+	 * accesses to the 'entries' array above index 0 so don't do that.
+	 *
+	 * xfs_attr_leaf_name_local_t namelist;
+	 * xfs_attr_leaf_name_remote_t valuelist;
+	 */
 } xfs_attr_leafblock_t;
 
 /*