net: Mark functions as static in net/sunrpc/svc_xprt.c

Mark functions as static in net/sunrpc/svc_xprt.c because they are not
used outside this file.

This eliminates the following warning in net/sunrpc/svc_xprt.c:
net/sunrpc/svc_xprt.c:574:5: warning: no previous prototype for ‘svc_alloc_arg’ [-Wmissing-prototypes]
net/sunrpc/svc_xprt.c:615:18: warning: no previous prototype for ‘svc_get_next_xprt’ [-Wmissing-prototypes]
net/sunrpc/svc_xprt.c:694:6: warning: no previous prototype for ‘svc_add_new_temp_xprt’ [-Wmissing-prototypes]

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>