scripts/gdb/symbols: factor out get_vmlinux()

scripts/gdb/symbols: factor out get_vmlinux()

Patch series "scripts/gdb/symbols: determine KASLR offset on s390 during
early boot".

I noticed that debugging s390 early boot using the support I introduced in
commit 28939c3e9925 ("scripts/gdb/symbols: determine KASLR offset on
s390") does not work.

The reason is that decompressor does not provide the vmcoreinfo note, so
KASLR offset needs to be extracted in a different way, which this series
implements.  Patches 1-2 are trivial refactorings, and patch 3 is the
implementation.

This patch (of 3):

Move the code that determines the current vmlinux file into a separate
function.  It will be useful later in order to analyze the kernel image in
physical memory during s390 early boot.

Link: https://lkml.kernel.org/r/20250515155811.114392-1-iii@linux.ibm.com
Link: https://lkml.kernel.org/r/20250515155811.114392-2-iii@linux.ibm.com
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>