btrfs: remove superfluous return value check at btrfs_dio_iomap_begin()

In the if statement that checks the return value from
btrfs_check_data_free_space(), there's no point to check if 'ret' is not
zero in the else branch, since the main if branch checked that it's zero,
so in the else branch it necessarily has a non-zero value.

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