ipmi: use vzalloc instead of kmalloc for user creation

ipmi: use vzalloc instead of kmalloc for user creation

We met mulitple times of failure of staring bmc-watchdog,
due to the runtime memory allocation failure of order 4.

     bmc-watchdog: page allocation failure: order:4, mode:0x40cc0(GFP_KERNEL|__GFP_COMP), nodemask=(null),cpuset=/,mems_allowed=0-1
     CPU: 1 PID: 2571 Comm: bmc-watchdog Not tainted 5.5.0-00045-g7d6bb61d6188c #1
     Hardware name: Intel Corporation S2600WFT/S2600WFT, BIOS SE5C620.86B.00.01.0015.110720180833 11/07/2018
     Call Trace:
      dump_stack+0x66/0x8b
      warn_alloc+0xfe/0x160
      __alloc_pages_slowpath+0xd3e/0xd80
      __alloc_pages_nodemask+0x2f0/0x340
      kmalloc_order+0x18/0x70
      kmalloc_order_trace+0x1d/0xb0
      ipmi_create_user+0x55/0x2c0 [ipmi_msghandler]
      ipmi_open+0x72/0x110 [ipmi_devintf]
      chrdev_open+0xcb/0x1e0
      do_dentry_open+0x1ce/0x380
      path_openat+0x305/0x14f0
      do_filp_open+0x9b/0x110
      do_sys_open+0x1bd/0x250
      do_syscall_64+0x5b/0x1f0
      entry_SYSCALL_64_after_hwframe+0x44/0xa9

Using vzalloc/vfree for creating ipmi_user heals the
problem

Thanks to Stephen Rothwell for finding the vmalloc.h
inclusion issue.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>