Both <linux/fs.h> and <sys/mount.h> define BLOCK_SIZE macros. Avoiding
using that name in block/migration.c.
I noticed this when including <liburing.h> (Linux io_uring) from
"block/aio.h" and compilation failed. Although patches adding that
include haven't been sent yet, it makes sense to rename the macro now in
case someone else stumbles on it in the meantime.