scripts/gdb: use mem instead of core_layout to get the module address

commit ac3b4328 ("module: replace module_layout with module_memory")
changed the struct module data structure from module_layout to
module_memory. The core_layout member which is used while loading
modules are not available anymore leading to the following error while
running gdb:

(gdb) lx-symbols
loading vmlinux
Python Exception <class 'gdb.error'>: There is no member named core_layout.
Error occurred in Python: There is no member named core_layout.

Replace core_layout with its new counterpart mem[MOD_TEXT].

Fixes: ac3b4328

 ("module: replace module_layout with module_memory")
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>