cifs: change cifs_get_name_from_search_buf to use new unicode helper

...and remove cifs_convertUCSpath. There are no more callers. Also add a
#define for the buffer used in the readdir path so that we don't have so
many magic numbers floating around.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>