fix #5409: backup: fix copy-before-write timeout

fix #5409: backup: fix copy-before-write timeout

The type for the copy-before-write timeout in nanoseconds was wrong.
By being just uint32_t, a maximum of slightly over 4 seconds was
possible. Larger values would overflow and thus the 45 seconds set by
Proxmox's backup with fleecing, resulted in effectively 2 seconds
timeout for copy-before-write operations.

Reported-by: Friedrich Weber <f.weber@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>