listxattr() sized its buffer with min(), which truncates the 64-bit
size to 32 bits, so a size of 2^32 + k allocated k bytes while the
copy-out guard used the full size. The walk then read record lengths
past the buffer and copied kernel heap out to the caller, and could
panic the kernel. Any user can reach it through a user.* attribute on
a file it owns.

Use MIN(), and fail with EIO if a record runs past the data returned.