Define smc_socket security class.

Linux kernel commit da69a5306ab9 ("selinux: support distinctions among all
network address families") triggers a build error if a new address family
is added without defining a corresponding SELinux security class.  As a
result, the smc_socket class was added to the kernel to resolve a build
failure as part of merge commit 3051bf36c25d that introduced AF_SMC circa
Linux 4.11.  Define this security class and its access vector, add
it to the socket_class_set macro, and exclude it from webview_zygote
like other socket classes.

Test:  Policy builds

Change-Id: Idbb8139bb09c6d1c47f1a76bd10f4ce1e9d939cb
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>