btrfs: remove redundant value assignment at btrfs_add_extent_mapping()

At btrfs_add_extent_mapping(), in case add_extent_mapping() returned
-EEXIST, it's pointless to assign 0 to 'ret' since we will assign a value
to it shortly after, without 'ret' being used before that. So remove that
pointless assignment.

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