systemtap: Fixed probe syscall.sendfile failure

compat_sendfile syscall is unavailable in kernels starting version 3.8.
Hence systemtap scripts like 'probe syscall.* { if (target()==pid()) log(name." ".argstr) }' fail.
This problem is solved by marking __syscall.compat_sendfile as optional in tapset/linux/syscalls2.stp

Upstream-Status: Pending

Signed-off-by: Mikhail Durnev <mikhail_durnev@mentor.com>

diff -Naur old/tapset/linux/syscalls2.stp new/tapset/linux/syscalls2.stp
--- old/tapset/linux/syscalls2.stp	2014-04-21 01:28:41.000000000 -0500
+++ new/tapset/linux/syscalls2.stp	2014-04-21 01:38:14.961233914 -0500
@@ -1953,7 +1953,7 @@
 # COMPAT_SYSCALL_DEFINE4(sendfile64, int, out_fd, int, in_fd,
 #		compat_loff_t __user *, offset, compat_size_t, count)
 #
-probe syscall.sendfile = __syscall.sendfile, __syscall.compat_sendfile
+probe syscall.sendfile = __syscall.sendfile, __syscall.compat_sendfile ?
 {
 	name = "sendfile"
 	out_fd = __int32($out_fd)