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 <[e-mail]>
Reviewed-by: Christian Brauner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/pidfd/pidfd.h

···
 	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

···
 #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;