btrfs: zoned: fix missing endianness conversion in sb_write_pointer

generation is an on-disk __le64 value, so use btrfs_super_generation to
convert it to host endian before comparing it.

Fixes: 12659251 ("btrfs: implement log-structured superblock for ZONED mode")
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>