x86/mce: fix misleading indentation in init_nonfatal_mce_checker()

x86/mce: fix misleading indentation in init_nonfatal_mce_checker()

Debian bug 812166[0] reported this build failure due to
Wmisleading-indentation with gcc-6:

non-fatal.c: In function 'init_nonfatal_mce_checker':
non-fatal.c:103:2: error: statement is indented as if it were guarded by... [-Werror=misleading-indentation]
  switch (c->x86_vendor) {
  ^~~~~~

non-fatal.c:97:5: note: ...this 'if' clause, but it is not
     if ( __get_cpu_var(poll_bankmask) == NULL )
     ^~

I was unable to reproduce (xen builds cleanly for me with "6.0.0 20160117
(experimental) [trunk revision 232481]") but looking at the code the issue
above is clearly real.

Correctly reindent the if statement.

This file uses Linux coding style (infact the use of Xen style for
this line is the root cause of the wanring) so use tabs and while
there remove the whitespace inside the if as Linux does.

[0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=812166

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>