ovl: Use "buf" flexible array for memcpy() destination

In the Linux kernel, the following vulnerability has been resolved: ovl: Use "buf" flexible array for memcpy() destination The "buf" flexible array needs to be the memcpy() destination to avoid false positive run-time warning from the recent FORTIFY_SOURCE hardening: memcpy: detected field-spanning write (size 93) of single field "&fh->fb" at fs/overlayfs/export.c:799 (size 21)