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.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>