qede: fix off-by-one in BD ring consumption on build_skb failure

In the Linux kernel, the following vulnerability has been resolved: qede: fix off-by-one in BD ring consumption on build_skb failure qede_rx_build_skb() and qede_tpa_rx_build_skb() do not check for a NULL return from qede_build_skb(). When it returns NULL under memory pressure, the functions still consume a BD from the ring before returning NULL. The callers then recycle additional BDs, resulting in one extra BD being consumed (off-by-one). This desynchronizes the BD ring, which can corrupt DMA page reference counts and lead to SLUB freelist corruption. Commit 4e910dbe3650 ("qede: confirm skb is allocated before using") added a NULL check inside qede_build_skb() to prevent a NULL pointer dereference, but did not address the missing NULL checks in the callers, making this off-by-one reachable. Fix this by adding NULL checks for the return value of qede_build_skb() in both qede_rx_build_skb() and qede_tpa_rx_build_skb(), returning NULL immediately before any BD ring manipulation.

Quoted source text, attributed separately from HOL analysis.