aufs: reduce overhead for "code present but disabled" use case.

The AUFS code introduces a new mm/prfile.c which is used to hook in a
shadow structure plus some __FUNC__ and __LINE__ tracing via hijacking
some fs/file and mm/vma calls (see aufs5-standalone/aufs5-mmap.patch).

As these tiny stubs live in a separate source module, we are always going
to be bouncing through them, even with AUFS completely disabled.  We also
see AUFS adding some content to existing structs even with AUFS disabled.

We can fix the 1st by getting rid of prfile.c in the disabled case; we
replace it with macros that collapse to the original unpatched code.

We can fix the latter by using IS_ENABLED(CONFIG_AUFS_FS) around the
extra content AUFS adds/changes so they aren't present when disabled.
This covers both the CONFIG_AUFS_FS=m and CONFIG_AUFS_FS=y cases.

Finally, there is the deprecated remap_file_pages() syscall - and while
it has been deprecated since 2014 - it does add unused code and possibly
exposure to regressions/CVE etc. - and so I also used the very same
IS_ENABLED() there so we have the original code in the disabled case.

The size difference for the disabled case is easy to detect/display.
Functions found in prfile.c show up as "-" below since it isn't built:

  linux-yocto$ size ../aufs*/vmlinux
     text    data     bss     dec     hex filename
  20176019        8124604 2019564 30320187        1cea63b ../aufs-debloat/vmlinux
  20176259        8124604 2019564 30320427        1cea72b ../aufs-default/vmlinux

  linux-yocto$./scripts/bloat-o-meter ../aufs-default/vmlinux ../aufs-debloat/vmlinux
  add/remove: 0/4 grow/shrink: 0/14 up/down: 0/-871 (-871)
  Function                                     old     new   delta
  filemap_page_mkwrite                         252     247      -5
  gate_vma                                     200     192      -8
  mmap_region                                 1528    1517     -11
  __vma_adjust                                2357    2345     -12
  remove_vma                                   104      91     -13
  dup_mm.isra                                 1403    1387     -16
  map_files_get_link                           429     408     -21
  copy_vma                                     597     576     -21
  __split_vma                                  420     394     -26
  show_numa_map                                944     917     -27
  vma_do_pr_or_file                             29       -     -29
  vma_do_get_file                               35       -     -35
  show_map_vma                                 350     310     -40
  vma_do_fput                                   47       -     -47
  vma_do_file_update_time                       47       -     -47
  __func__                                   17583   17424    -159
  __ia32_sys_remap_file_pages                  786     623    -163
  __x64_sys_remap_file_pages                   817     626    -191
  Total: Before=17285664, After=17284793, chg -0.01%
  linux-yocto$

Performance should be restored to the original non-AUFS baseline given
the above reduction in code and removal of out-of-line stubs.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>