fs/proc/vmcore: a few cleanups for vmcore_add_device_dump()

fs/proc/vmcore: a few cleanups for vmcore_add_device_dump()

There are two cleanups for vmcore_add_device_dump().  Return -ENOMEM
directly rather than goto the label to simplify the code and use
scoped_guard() to simplify the lock/unlock code.

Link: https://lkml.kernel.org/r/20250626105440.1053139-1-suhui@nfschina.com
Signed-off-by: Su Hui <suhui@nfschina.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Suhui <suhui@nfschina.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>