NFSv4: rpc_mkpipe creating socket inodes w/out sk buffers

This patch stop rpc_mkpipe from create S_IFSOCK nodes what don't
have associated sk buffers attached (which causes SELinux to oops
during NFSv4 mounts). Instead the S_IFIFO mode bit is set which
probably make more sense and seems to work just fine during
my connectathon and fsx testing...

Signed-off-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>