CVE-2026-53250: xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

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

xsk: cache csumstart/csumoffset to fix TOCTOU in xskskbmetadata()

The TX metadata area resides in the UMEM buffer which is memory-mapped and concurrently writable by userspace. In xskskbmetadata(), csumstart and csumoffset are read from shared memory for bounds validation, then read again for skb assignment. A malicious userspace application can race to overwrite these values between the two reads, bypassing the bounds check and causing out-of-bounds memory access during checksum computation in the transmit path.

Fix this by reading csumstart and csumoffset into local variables once, then using the local copies for both validation and assignment.

Note that other metadata fields (flags, launchtime) and the cached csum fields may be mutually inconsistent due to concurrent userspace writes, but this is benign: the only security-critical invariant is that each field's validated value is the same one used, which local caching guarantees.