cifs: fix buffer size for tcon->nativeFileSystem field

The buffer for this was resized recently to fix a bug. It's still
possible however that a malicious server could overflow this field
by sending characters in it that are >2 bytes in the local charset.
Double the size of the buffer to account for this possibility.

Also get rid of some really strange and seemingly pointless NULL
termination. It's NULL terminating the string in the source buffer,
but by the time that happens, we've already copied the string.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>