x86, msr: Use file_inode(), not f_mapping->host

As discussed in [1], exchange f_mapping->host with file_inode().  This
is a bug, but happens to be non-manifest in this case.

[1] http://lkml.kernel.org/r/20131007190357.GA13318@ZenIV.linux.org.ukSigned-off-by: Andre Richter <andre.o.richter@gmail.com>
Link: http://lkml.kernel.org/r/1381224142-3267-1-git-send-email-andre.o.richter@gmail.comSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/msr.c

@@ -46,7 +46,7 @@ static struct class *msr_class;
 static loff_t msr_seek(struct file *file, loff_t offset, int orig)
 {
 	loff_t ret;
-	struct inode *inode = file->f_mapping->host;
+	struct inode *inode = file_inode(file);
 
 	mutex_lock(&inode->i_mutex);
 	switch (orig) {