x86/mce: Fix build bug with CONFIG_PROVE_LOCKING=y && CONFIG_X86_MCE_INTEL=y

Commit f56e8a07 "x86/mce: Fix RCU lockdep splats" introduced the
following build bug:

  arch/x86/kernel/cpu/mcheck/mce.c: In function 'mce_log':
  arch/x86/kernel/cpu/mcheck/mce.c:166: error: 'mce_read_mutex' undeclared (first use in this function)
  arch/x86/kernel/cpu/mcheck/mce.c:166: error: (Each undeclared identifier is reported only once
  arch/x86/kernel/cpu/mcheck/mce.c:166: error: for each function it appears in.)

Move the in-the-middle-of-file lock variable up to the variable
definition section, the top of the .c file.

Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1267830207-9474-3-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>