x86/mtrr: Don't copy out-of-bounds data in mtrr_write

Don't access the provided buffer out of bounds - this can cause a kernel
out-of-bounds read when invoked through sys_splice() or other things that
use kernel_write()/__kernel_write().

Fixes: 7f8ec5a4

 ("x86/mtrr: Convert to use strncpy_from_user() helper")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180706215003.156702-1-jannh@google.com