btrfs: use a single variable for return value at run_delayed_extent_op()

Instead of using a 'ret' and an 'err' variable at run_delayed_extent_op()
for tracking the return value, use a single one ('ret'). This simplifies
the code, makes it comply with most of the existing code and it's less
prone for logic errors as time has proven over and over in the btrfs code.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>