btrfs: fix double put of block group with nocow

While debugging a patch that I wrote I was hitting use-after-free panics
when accessing block groups on unmount.  This turned out to be because
in the nocow case if we bail out of doing the nocow for whatever reason
we need to call btrfs_dec_nocow_writers() if we called the inc.  This
puts our block group, but a few error cases does

if (nocow) {
    btrfs_dec_nocow_writers();
    goto error;
}

unfortunately, error is

error:
	if (nocow)
		btrfs_dec_nocow_writers();

so we get a double put on our block group.  Fix this by dropping the
error cases calling of btrfs_dec_nocow_writers(), as it's handled at the
error label now.

Fixes: 762bf09 ("btrfs: improve error handling in run_delalloc_nocow")
CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -1690,12 +1690,8 @@ static noinline int run_delalloc_nocow(struct inode *inode,
 			ret = fallback_to_cow(inode, locked_page, cow_start,
 					      found_key.offset - 1,
 					      page_started, nr_written);
-			if (ret) {
-				if (nocow)
-					btrfs_dec_nocow_writers(fs_info,
-								disk_bytenr);
+			if (ret)
 				goto error;
-			}
 			cow_start = (u64)-1;
 		}
 
@@ -1711,9 +1707,6 @@ static noinline int run_delalloc_nocow(struct inode *inode,
 					  ram_bytes, BTRFS_COMPRESS_NONE,
 					  BTRFS_ORDERED_PREALLOC);
 			if (IS_ERR(em)) {
-				if (nocow)
-					btrfs_dec_nocow_writers(fs_info,
-								disk_bytenr);
 				ret = PTR_ERR(em);
 				goto error;
 			}