net: Mark function as static in 9p/client.c

Mark function as static in net/9p/client.c because it is not used
outside this file.

This eliminates the following warning in net/9p/client.c:
net/9p/client.c:207:18: warning: no previous prototype for ‘p9_fcall_alloc’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/client.c

@@ -204,7 +204,7 @@ static int parse_opts(char *opts, struct p9_client *clnt)
 	return ret;
 }
 
-struct p9_fcall *p9_fcall_alloc(int alloc_msize)
+static struct p9_fcall *p9_fcall_alloc(int alloc_msize)
 {
 	struct p9_fcall *fc;
 	fc = kmalloc(sizeof(struct p9_fcall) + alloc_msize, GFP_NOFS);