exec: fix off-by-one in binfmt max rewrite depth comment

exec: fix off-by-one in binfmt max rewrite depth comment

The loop in exec_binprm() permits depth values 0 through 5, up to 5
successive binfmt rewrites (setting bprm->interpreter) until the 6th
one would fail on depth > 5 and return -ELOOP. The comment claimed 4
levels, which was wrong. Adjusting the code to allow only 4 rewrites
would be breaking userland, so fix the comment and not the code.

Reproducer (a chain of shebanged scripts followed by an ELF binary):

    #!/bin/sh

    tmp=$(mktemp -d)
    echo $tmp
    cd $tmp

    mk () { echo $2 > $1; chmod +x $1; }

    for i in $(seq 4); do
    	mk $i "#!$((i + 1))"
    done

    mk 5 '#!/bin/true'
    ./1 &&
    echo '5 binfmt rewrites OK (1 -> 2 -> 3 -> 4 -> 5 -> /bin/true)'

    mk 5 '#!6'
    mk 6 '#!/bin/true'
    ./1 ||
    echo '6 binfmt rewrites KO (1 -> 2 -> 3 -> 4 -> 5 -> 6 -> /bin/true)'

Signed-off-by: Alan Urmancheev <alan.urman@gmail.com>
Link: https://patch.msgid.link/20260623052322.74711-1-alan.urman@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>