ext4: fix ext4_free_inode() vs. ext4_claim_inode() race

I was seeing fsck errors on inode bitmaps after a 4 thread
dbench run on a 4 cpu machine:

Inode bitmap differences: -50736 -(50752--50753) etc...

I believe that this is because ext4_free_inode() uses atomic
bitops, and although ext4_new_inode() *used* to also use atomic 
bitops for synchronization, commit 
39341867 changed this to use
the sb_bgl_lock, so that we could also synchronize against
read_inode_bitmap and initialization of uninit inode tables.

However, that change left ext4_free_inode using atomic bitops,
which I think leaves no synchronization between setting & 
unsetting bits in the inode table.

The below patch fixes it for me, although I wonder if we're 
getting at all heavy-handed with this spinlock...
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ialloc.c

@@ -188,7 +188,7 @@ void ext4_free_inode(handle_t *handle, struct inode *inode)
 	struct ext4_group_desc *gdp;
 	struct ext4_super_block *es;
 	struct ext4_sb_info *sbi;
-	int fatal = 0, err, count;
+	int fatal = 0, err, count, cleared;
 	ext4_group_t flex_group;
 
 	if (atomic_read(&inode->i_count) > 1) {
@@ -248,8 +248,10 @@ void ext4_free_inode(handle_t *handle, struct inode *inode)
 		goto error_return;
 
 	/* Ok, now we can actually update the inode bitmaps.. */
-	if (!ext4_clear_bit_atomic(sb_bgl_lock(sbi, block_group),
-					bit, bitmap_bh->b_data))
+	spin_lock(sb_bgl_lock(sbi, block_group));
+	cleared = ext4_clear_bit(bit, bitmap_bh->b_data);
+	spin_unlock(sb_bgl_lock(sbi, block_group));
+	if (!cleared)
 		ext4_error(sb, "ext4_free_inode",
 			   "bit already cleared for inode %lu", ino);
 	else {