x86/mtrr: Convert to use strncpy_from_user() helper

Replace the open coded string fetch from user-space with strncpy_from_user().
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Toshi Kani <toshi.kani@hp.com>
Link: http://lkml.kernel.org/r/20180515180535.89703-1-andriy.shevchenko@linux.intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/mtrr/if.c

@@ -106,17 +106,9 @@ mtrr_write(struct file *file, const char __user *buf, size_t len, loff_t * ppos)
 
 	memset(line, 0, LINE_SIZE);
 
-	length = len;
-	length--;
-
-	if (length > LINE_SIZE - 1)
-		length = LINE_SIZE - 1;
-
+	length = strncpy_from_user(line, buf, LINE_SIZE - 1);
 	if (length < 0)
-		return -EINVAL;
-
-	if (copy_from_user(line, buf, length))
-		return -EFAULT;
+		return length;
 
 	linelen = strlen(line);
 	ptr = line + linelen - 1;