cifs: rename cifs_common to smbfs_common

As we move to common code between client and server, we have
been asked to make the names less confusing, and refer less
to "cifs" and more to words which include "smb" instead to
e.g. "smbfs" for the client (we already have "ksmbd" for the
kernel server, and "smbd" for the user space Samba daemon).
So to be more consistent in the naming of common code between
client and server and reduce the risk of merge conflicts as
more common code is added - rename "cifs_common" to
"smbfs_common" (in future releases we also will rename
the fs/cifs directory to fs/smbfs)

Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>