btrfs: remove err variable from btrfs_delete_subvolume

[ Upstream commit ee0d904f ]

Use only a single 'ret' to control whether we should abort the
transaction or not. That's fine, because if we abort a transaction then
btrfs_end_transaction will return the same value as passed to
btrfs_abort_transaction. No semantic changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 3324d054 ("btrfs: avoid copying BTRFS_ROOT_SUBVOL_DEAD flag to snapshot of subvolume being deleted")
Signed-off-by: Sasha Levin <sashal@kernel.org>