cifs.upcall: allocate a talloc context for smb_krb5_unparse_name

cifs.upcall: allocate a talloc context for smb_krb5_unparse_name

cifs.upcall calls smb_krb5_unparse_name with a NULL talloc context.
Older versions of this function though will conditionally use
SMB_REALLOC instead of TALLOC_REALLOC when a NULL context is passed
in. To make it more consistent, just spawn a talloc context that
we can pass into this function.

Resolves:
https://bugzilla.redhat.com/show_bug.cgi?id=565446
https://bugzilla.samba.org/show_bug.cgi?id=6868

Reported-by: Ludek Finstrle <luf@seznam.cz>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Günther Deschner <gd@samba.org>