binfmt_misc: let bpf handlers pass an argument to the interpreter

A bpf binfmt_misc handler selects an interpreter but, unlike binfmt_script,
load_misc_binary() builds the argument vector as just [interpreter, binary,
...] with no slot for an argument to the interpreter. A handler that wants
to reproduce a #! line therefore cannot express its single optional
argument, e.g. a handler that resolves $ORIGIN in a script's #! path loses
the argument that followed the interpreter.

Have load_misc_binary() consume the argument staged through the
bpf_binprm_set_interp_arg() kfunc and insert it between the interpreter and
the binary - the same position and single-argument semantics binfmt_script
gives the argument of a #! line. The argument is cleared once spliced into
the argument vector, and a load program that fails after staging one has it
dropped on the way out: whether the exec fails or -ENOEXEC hands the binary
back to the remaining formats, a stale argument cannot leak into a nested
interpreter's argv. This also lets static-style handlers pass a fixed
interpreter argument, which plain binfmt_misc has never been able to
express.

Link: https://patch.msgid.link/20260714-work-bpf-binfmt_misc-v2-6-57b7529c002c@kernel.org
Reviewed-by: Farid Zakaria <farid.m.zakaria@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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