locks: report l_pid as -1 for FL_FILE_PVT locks

FL_FILE_PVT locks are no longer tied to a particular pid, and are
instead inheritable by child processes. Report a l_pid of '-1' for
these sorts of locks since the pid is somewhat meaningless for them.

This precedent comes from FreeBSD. There, POSIX and flock() locks can
conflict with one another. If fcntl(F_GETLK, ...) returns a lock set
with flock() then the l_pid member cannot be a process ID because the
lock is not held by a process as such.
Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@redhat.com>

fs/locks.c

@@ -1864,7 +1864,7 @@ EXPORT_SYMBOL_GPL(vfs_test_lock);
 
 static int posix_lock_to_flock(struct flock *flock, struct file_lock *fl)
 {
-	flock->l_pid = fl->fl_pid;
+	flock->l_pid = IS_FILE_PVT(fl) ? -1 : fl->fl_pid;
 #if BITS_PER_LONG == 32
 	/*
 	 * Make sure we can represent the posix lock via
@@ -1886,7 +1886,7 @@ static int posix_lock_to_flock(struct flock *flock, struct file_lock *fl)
 #if BITS_PER_LONG == 32
 static void posix_lock_to_flock64(struct flock64 *flock, struct file_lock *fl)
 {
-	flock->l_pid = fl->fl_pid;
+	flock->l_pid = IS_FILE_PVT(fl) ? -1 : fl->fl_pid;
 	flock->l_start = fl->fl_start;
 	flock->l_len = fl->fl_end == OFFSET_MAX ? 0 :
 		fl->fl_end - fl->fl_start + 1;