cifs: Move some extern decls from .c files to .h

Move the following:

        extern mempool_t *cifs_sm_req_poolp;
        extern mempool_t *cifs_req_poolp;
        extern mempool_t *cifs_mid_poolp;
        extern bool disable_legacy_dialects;

from various .c files to cifsglob.h.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>