cifs: Fix creating native symlinks pointing to current or parent directory

cifs: Fix creating native symlinks pointing to current or parent directory

Calling 'ln -s . symlink' or 'ln -s .. symlink' creates symlink pointing to
some object name which ends with U+F029 unicode codepoint. This is because
trailing dot in the object name is replaced by non-ASCII unicode codepoint.

So Linux SMB client currently is not able to create native symlink pointing
to current or parent directory on Windows SMB server which can be read by
either on local Windows server or by any other SMB client which does not
implement compatible-reverse character replacement.

Fix this problem in cifsConvertToUTF16() function which is doing that
character replacement. Function comment already says that it does not need
to handle special cases '.' and '..', but after introduction of native
symlinks in reparse point form, this handling is needed.

Note that this change depends on the previous change
"cifs: Improve creating native symlinks pointing to directory".

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>