CVE-2026-68186 binfmt_misc: set have_execfd only once the interpreter is opened

In the Linux kernel, the following vulnerability has been resolved: binfmtmisc: set haveexecfd only once the interpreter is opened loadmiscbinary raises bprm-haveexecfd as soon as it sees the 'O' or 'C' flag. This happens well before it opens the interpreter. If that open fails the flag stays set...