cifs: Check for UTF-16 null codepoint in SFU symlink target location

Check that read buffer of SFU symlink target location does not contain
UTF-16 null codepoint (via UniStrnlen() call) because Linux cannot process
symlink with null byte, it truncates everything in buffer after null byte.

Fixes: cf2ce673 ("cifs: Add support for reading SFU symlink location")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>