tracing, Text Edit Lock - kprobes architecture independent support, nommu fix

Impact: build fix on SH !CONFIG_MMU

Stephen Rothwell reported this linux-next build failure on the SH
architecture:

  kernel/built-in.o: In function `disable_all_kprobes':
  kernel/kprobes.c:1382: undefined reference to `text_mutex'
  [...]

And observed:

| Introduced by commit 4460fdad85 ("tracing,
| Text Edit Lock - kprobes architecture independent support") from the
| tracing tree.  text_mutex is defined in mm/memory.c which is only built
| if CONFIG_MMU is defined, which is not true for sh allmodconfig.

Move this lock to kernel/extable.c (which is already home to various
kernel text related routines), which file is always built-in.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
LKML-Reference: <20090320110602.86351a91.sfr@canb.auug.org.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

mm/memory.c

@@ -101,14 +101,6 @@ int randomize_va_space __read_mostly =
 					2;
 #endif
 
-/*
- * mutex protecting text section modification (dynamic code patching).
- * some users need to sleep (allocating memory...) while they hold this lock.
- *
- * NOT exported to modules - patching kernel text is a really delicate matter.
- */
-DEFINE_MUTEX(text_mutex);
-
 static int __init disable_randmaps(char *s)
 {
 	randomize_va_space = 0;