btrfs: simplify add_extent_mapping() by removing pointless label

btrfs: simplify add_extent_mapping() by removing pointless label

The add_extent_mapping() function is short and trivial, there's no need to
have a label for a quick exit in case of an error, even because there's no
error handling needed, we just need to return the error. So remove that
label and return directly.

Also while at it remove the redundant initialization of 'ret', as that may
help avoid some warnings with clang tools such as the one reported/fixed
by commit 966de47ff0c9 ("btrfs: remove redundant initialization of
variables in log_new_ancestors").

Reviewed-by: Qu Wenruo <wqu@suse.com>
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>