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

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.

Fixes: 8a8633978b84 ("qede: Add build_skb() support.")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Reviewed-by: Jamie Bainbridge <jamie.bainbridge@gmail.com>
Link: https://patch.msgid.link/20260630164623.3152625-1-syoshida@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>