exfat: allow access to paths with trailing dots

 The Linux kernel exfat driver currently unconditionally strips
 trailing periods '.' from path components. This isdone intentionally,
 loosely following Windows behaviour and specifications
 which state:

  #exFAT
  The concatenated file name has the same set of illegal characters as
  other FAT-based file systems (see Table 31).

  #FAT
  ...
  Leading and trailing spaces in a long name are ignored.
  Leading and embedded periods are allowed in a name and are stored in
  the long name. Trailing periods are ignored.

Note: Leading and trailing space ' ' characters are currently retained
by Linux kernel exfat, in conflict with the above specification.
On Windows 10, trailing and leading space ' ' characters are stripped
from the filenames.
Some implementations, such as fuse-exfat, don't perform path trailer
removal. When mounting images which contain trailing-dot paths, these
paths are unreachable, e.g.:

  + mount.exfat-fuse /dev/zram0 /mnt/test/
  FUSE exfat 1.3.0
  + cd /mnt/test/
  + touch fuse_created_dots... '  fuse_created_spaces  '
  + ls -l
  total 0
  -rwxrwxrwx 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
  -rwxrwxrwx 1 root 0 0 Aug 18 09:45  fuse_created_dots...
  + cd /
  + umount /mnt/test/
  + mount -t exfat /dev/zram0 /mnt/test
  + cd /mnt/test
  + ls -l
  ls: cannot access 'fuse_created_dots...': No such file or directory
  total 0
  -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
  -????????? ? ?    ? ?            ?  fuse_created_dots...
  + touch kexfat_created_dots... '  kexfat_created_spaces  '
  + ls -l
  ls: cannot access 'fuse_created_dots...': No such file or directory
  total 0
  -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  fuse_created_spaces  '
  -rwxr-xr-x 1 root 0 0 Aug 18 09:45 '  kexfat_created_spaces  '
  -????????? ? ?    ? ?            ?  fuse_created_dots...
  -rwxr-xr-x 1 root 0 0 Aug 18 09:45  kexfat_created_dots
  + cd /
  + umount /mnt/test/

This commit adds "keep_last_dots" mount option that controls whether or
not trailing periods '.' are stripped
from path components during file lookup or file creation.
This mount option can be used to access
paths with trailing periods and disallow creating files with names with
trailing periods. E.g. continuing from the previous example:

  + mount -t exfat -o keep_last_dots /dev/zram0 /mnt/test
  + cd /mnt/test
  + ls -l
  total 0
  -rwxr-xr-x 1 root 0 0 Aug 18 10:32 '  fuse_created_spaces  '
  -rwxr-xr-x 1 root 0 0 Aug 18 10:32 '  kexfat_created_spaces  '
  -rwxr-xr-x 1 root 0 0 Aug 18 10:32  fuse_created_dots...
  -rwxr-xr-x 1 root 0 0 Aug 18 10:32  kexfat_created_dots

  + echo > kexfat_created_dots_again...
  sh: kexfat_created_dots_again...: Invalid argument

Link: https://bugzilla.suse.com/show_bug.cgi?id=1188964
Link: https://lore.kernel.org/linux-fsdevel/003b01d755e4$31fb0d80$95f12880$
@samsung.com/
Link: https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification


Suggested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Vasant Karasulli <vkarasulli@suse.de>
Co-developed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>