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.uk

Signed-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.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>