x86/mm: Rearrange guest_get_eff_{,kern_}l1e() to not be void

x86/mm: Rearrange guest_get_eff_{,kern_}l1e() to not be void

Coverity complains that gl1e.l1 may be used while uninitialised in
map_ldt_shadow_page().  This isn't actually accurate as guest_get_eff_l1e()
will always write to its parameter.

However, having a void function which returns a 64bit value via pointer is
rather silly.  Rearrange the functions to return l1_pgentry_t.

No functional change, but hopefully should help Coverity not to come to the
wrong conclusion.

Bloat-o-meter also reports a modest improvement:
  add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-71 (-71)
  function                                     old     new   delta
  guest_get_eff_l1e                             82      75      -7
  mmio_ro_do_page_fault                        530     514     -16
  map_ldt_shadow_page                          501     485     -16
  ptwr_do_page_fault                           615     583     -32

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>