kexec: fix crash_save_vmcoreinfo_init build problem

This fixes

  kernel/kexec.c: In function 'crash_save_vmcoreinfo_init':
  kernel/kexec.c:1374: error: 'vmlist' undeclared (first use in this function)
  kernel/kexec.c:1374: error: (Each undeclared identifier is reported only once
  kernel/kexec.c:1374: error: for each function it appears in.)
  kernel/kexec.c:1410: error: invalid use of undefined type 'struct vm_struct'
  make[1]: *** [kernel/kexec.o] Error 1

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>