remove spurious lock in popen

the newly allocated FILE * has not yet leaked to the application and
is only visible to stdio internals until popen returns. since we do
not change any fields of the structure observed by libc internals,
only the pipe_pid member, locking is not necessary.

src/stdio/popen.c

@@ -31,7 +31,6 @@ FILE *popen(const char *cmd, const char *mode)
 		__syscall(SYS_close, p[1]);
 		return NULL;
 	}
-	FLOCK(f);
 
 	e = ENOMEM;
 	if (!posix_spawn_file_actions_init(&fa)) {
@@ -43,7 +42,6 @@ FILE *popen(const char *cmd, const char *mode)
 				if (!strchr(mode, 'e'))
 					fcntl(p[op], F_SETFD, 0);
 				__syscall(SYS_close, p[1-op]);
-				FUNLOCK(f);
 				return f;
 			}
 		}