9p: Add mksock support

Without this patch, an attempt to mksock will get an EINVAL.

Before this patch:
[root@localhost 1dir]# mksock mysock
mksock: error making mysock: Invalid argument

With this patch:
[root@localhost 1dir]# mksock mysock
[root@localhost 1dir]# ls    -l mysock
s--------- 1 root root 0 2010-03-31 17:44 mysock
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

fs/9p/vfs_inode.c

@@ -1227,6 +1227,8 @@ v9fs_vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t rdev)
 		sprintf(name, "c %u %u", MAJOR(rdev), MINOR(rdev));
 	else if (S_ISFIFO(mode))
 		*name = 0;
+	else if (S_ISSOCK(mode))
+		*name = 0;
 	else {
 		__putname(name);
 		return -EINVAL;