net: wwan: mhi_wwan_mbim: check skb_copy_bits() return value

In the Linux kernel, the following vulnerability has been resolved: net: wwan: mhi_wwan_mbim: check skb_copy_bits() return value mhi_mbim_rx() ignores the return value of skb_copy_bits() when it copies each datagram out of the NTB. The datagram offset and length come from the DPE, which is only checked to lie within the NTB itself, so a modem can point a datagram outside the received skb. The copy then fails and the freshly allocated skbn is passed to netif_rx() with its uninitialized contents still in place, leaking kernel heap memory into the network stack. Free the skb and account an error when the copy fails. Verified in a QEMU guest with a fault injector pointing a DPE outside the received NTB: the copy fails, and the unpatched driver hands the uninitialized skbn to the network stack (observed as "unknown protocol" on bytes that were never written). With this check the failed datagram is dropped and counted as an rx error. Changes in v2: factor the free-and-count sequence out into mhi_mbim_rx_drop(), shared with the unknown-protocol path, as suggested by Loic Poulain.