CVE-2026-74608: smb: client: Fix use-after-free in cifs_try_adding_channels()

In the Linux kernel, the following vulnerability has been resolved:

smb: client: Fix use-after-free in cifstryaddingchannels()

cifstryaddingchannels() takes a temporary reference to an interface before dropping ifacelock. If cifssesaddchannel() fails, it drops that reference and then increments iface->weightfulfilled.

A concurrent interface list refresh can remove the list reference while channel creation is in progress. In that case, the failure-path krefput() releases the last reference and frees iface. Updating weightfulfilled afterward then accesses freed memory.

Increment weightfulfilled before dropping the temporary reference, keeping iface alive for the final access.