kernel/extable: fix address-checks for core_kernel and init areas

The init_kernel_text() and core_kernel_text() functions should not
include the labels _einittext and _etext when checking if an address is
inside the .text or .init sections.

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>