cifs: Add support for parsing WSL-style symlinks

cifs: Add support for parsing WSL-style symlinks

Linux CIFS client currently does not implement readlink() for WSL-style
symlinks. It is only able to detect that file is of WSL-style symlink, but
is not able to read target symlink location.

Add this missing functionality and implement support for parsing content of
WSL-style symlink.

The important note is that symlink target location stored for WSL symlink
reparse point (IO_REPARSE_TAG_LX_SYMLINK) is in UTF-8 encoding instead of
UTF-16 (which is used in whole SMB protocol and also in all other symlink
styles). So for proper locale/cp support it is needed to do conversion from
UTF-8 to local_nls.

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