CVE-2026-68151: binfmt_elf_fdpic: only honour the first PT_INTERP

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

binfmtelffdpic: only honour the first PTINTERP

The program header scan handles PTINTERP from a switch nested in the scan loop, so its break leaves the switch and not the loop. A binary carrying more than one PTINTERP runs the case again and overwrites both interpretername and interpreter. The previous name allocation leaks and so does the previous interpreter reference, along with the write denial openexec() took on it. The denial is never released, so the file stays unwritable for as long as the system runs.

An unprivileged caller reaches this with a crafted binary and repeats it at will. binfmtelf stops at the first PTINTERP. Do the same here.

The flaw dates back to the driver's introduction in the pre-git history tree introduced in v2.6.11 by 91808d6ebe39 ("[PATCH] FRV: Add FDPIC ELF binary format driver").