tests: move common definitions and functions into pidfd.h

Move definitions and functions used across different pidfd tests into
pidfd.h header.
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Christian Brauner <christian@brauner.io>
Link: https://lore.kernel.org/r/20190726162226.252750-1-surenb@google.comSigned-off-by: Christian Brauner <christian.brauner@ubuntu.com>

tools/testing/selftests/pidfd/pidfd.h

@@ -73,6 +73,11 @@ int wait_for_pid(pid_t pid)
 	return WEXITSTATUS(status);
 }
 
+static inline int sys_pidfd_open(pid_t pid, unsigned int flags)
+{
+	return syscall(__NR_pidfd_open, pid, flags);
+}
+
 static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
 					unsigned int flags)
 {

tools/testing/selftests/pidfd/pidfd_open_test.c

@@ -22,11 +22,6 @@
 #include "pidfd.h"
 #include "../kselftest.h"
 
-static inline int sys_pidfd_open(pid_t pid, unsigned int flags)
-{
-	return syscall(__NR_pidfd_open, pid, flags);
-}
-
 static int safe_int(const char *numstr, int *converted)
 {
 	char *err = NULL;