btrfs: replace unnecessary goto with direct return at cow_file_range()

The 'goto out' in cow_file_range() in the exit block are not necessary
and jump back. Replace them with return, while still keeping 'goto out'
in the main code.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ keep goto in the main code, update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>