md: add a missing endianness conversion in check_sb_changes

The on-disk value is little endian and we need to convert it to
native endian before storing the value in the in-core structure.

Fixes: 7564beda ("md-cluster/raid10: support add disk under grow mode")
Cc: <stable@vger.kernel.org> # 4.20+
Acked-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <songliubraving@fb.com>