btrfs-progs: convert: Fix offset-by-one error in read_data_extent()

btrfs-progs: convert: Fix offset-by-one error in read_data_extent()

For read_data_extent() in convert/main.c it's using mirror number in a
incorrect way, which will not get correct copy for RAID1:

	for (cur_mirror = 0; cur_mirror < num_copies; cur_mirror++) {

In such case, for RAID1 @cur_mirror will only be 0 and 1.

However for 0 and 1 case, btrfs_map_block() will only return the first
copy.  To reach the 2nd copy, it correct @cur_mirror range should be 1
and 2.

So with this off-by-one error, btrfs-image will never be able to read
out data extent if the first stripe of the chunk is the missing one.

Fix it by starting @cur_mirror from 1 and to @num_copies (including).

Fixes: 2d46558b30f5 ("btrfs-progs: Use existing facility to replace read_data_extent function")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>