compel handle_elf(): fix strings sect bounds check

Got this when using compel hgen on arm 32-bit:

Error (compel/src/lib/handle-elf-host.c:115): String section header
@0xf66e11ec is out of [0xf66e1174, 0xf66e1264)

Looking at this, it does not make sense. For the reference,
sizeof(Elf_Shdr) is 0x28, so end position is also well within bounds.

Apparently, the check for string section header bounds is wrong
as the last argument of __ptr_struct_oob() is supposed to be a
region size, not the region end address as it is.

This always worked before because the check was too relaxed, and
compel was never used on 32-bit ARM. This time it didn't work
because of a 32-bit overflow, which helped to find this bug.

This is a fix to commit 6402f03 ("compel: separate get_strings_section
from __handle_elf").

Cc: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>