extable: Make init_kernel_text() global

Convert init_kernel_text() to a global function and use it in a few
places instead of manually comparing _sinittext and _einittext.

Note that kallsyms.h has a very similar function called
is_kernel_inittext(), but its end check is inclusive.  I'm not sure
whether that's intentional behavior, so I didn't touch it.

Suggested-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/4335d02be8d45ca7d265d2f174251d0b7ee6c5fd.1519051220.git.jpoimboe@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>