btrfs: consolidate the error handling in run_delalloc_nocow

Share the calls to extent_clear_unlock_delalloc for btrfs_path allocation
failure handling and the normal exit path.

This relies on btrfs_free_path ignoring a NULL pointer, and the
initialization of cur_offset to start at the beginning of the function.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>