inotify: fix compatibillity with address + netmask separate passed

inotify: fix compatibillity with address + netmask separate passed

fixes commit 78c6656c9aba1d57786f916717c2622a3059fb6 which dropped
writing out the netmask but missed to add compat code for the case
where the caller did not suplly a address in CIDR format already.

Check if an address ends with /\d+, if not see if a netmask is
available and add that similar how it gets handled on read.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Reviewed-By: Stoiko Ivanov <s.ivanov@proxmox.com>
Tested-By: Stoiko Ivanov <s.ivanov@proxmox.com>