btrfs-progs: mkfs: return nozero value on thin provisioned device

btrfs-progs: mkfs: return nozero value on thin provisioned device

With mkfs.btrfs on a thin provisioned device with very small backing
size and big virtual size, all code works well in mkfs.btrfs until
close_ctree() is called.
close_ctree() fails to sync device due to small backing size while
closing devices.  However, mkfs returns 0 in such situation which causes
failure of fstests generic/405.

So, let mkfs returns nonzero value if previous steps succeeded but
close_ctree() failed. Then fstests generic/405 passes now.

Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>