librbd: Endian fix for handling old image format resize requests

librbd: Endian fix for handling old image format resize requests

ResizeRequest<I>::send_update_header contains a comment
    // NOTE: format 1 image headers are not stored in fixed endian
format

This statement appears incorrect; Linux kernel code handling the old
format headers does assume they are in fixed little-endian format.

Fixed by performing a byte-swap as everywhere else; this actually
fixes test suite failures on big-endian systems.

Fixes (partially): https://tracker.ceph.com/issues/41605

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>