btrfs: free path sooner at __btrfs_unlink_inode()

btrfs: free path sooner at __btrfs_unlink_inode()

After calling btrfs_delete_one_dir_name() there's no need for the path
anymore so we can free it immediately after. After that point we do
some btree operations that take time and in those call chains we end up
allocating paths for these operations, so we're unnecessarily holding on
to the path we allocated early at __btrfs_unlink_inode().

So free the path as soon as we don't need it and add a comment. This
also allows to simplify the error path, removing two exit labels and
returning directly when an error happens.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>