CIFS: Adds information-level logging function

Currently, CIFS lacks a internal logging function that prints out data
when CIFS_DEBUG=n. When CIFS_DEBUG=y, the only message level for CIFS
events are KERN_ERR or KERN_DEBUG.

This patch creates cifs_info(), which is useful for printing
non-critical event messges, at either CIFS_DEBUG state.

Signed-off-by: Rodrigo Freire <rfreire@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>