btrfs: free path at can_nocow_extent() before checking for checksum items

When we look for checksum items, through csum_exist_in_range(), at
can_nocow_extent(), we no longer need the path that we have previously
allocated. Through csum_exist_in_range() -> btrfs_lookup_csums_range(),
we also end up allocating a path, so we are adding unnecessary extra
memory usage. So free the path before calling csum_exist_in_range().

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