Allow fcntl F_SETLK on sockets.

Addresses denials such as:
type=1400 : avc: denied { lock } for comm="PushCheckSendS" path="socket:[1834573]" dev="sockfs" ino=X scontext=u:r:untrusted_app:s0 tcontext=u:r:untrusted_app:s0 tclass=tcp_socket

Change-Id: Idbf6120cca9df634e2f8a876fd1cd836551e5ad7
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>