fix mremap memory synchronization and use of variadic argument

since mremap with the MREMAP_FIXED flag is an operation that unmaps
existing mappings, it needs to use the vm lock mechanism to ensure
that any in-progress synchronization operations using vm identities
from before the call have finished.

also, the variadic argument was erroneously being read even if the
MREMAP_FIXED flag was not passed. in practice this didn't break
anything, but it's UB and in theory LTO could turn it into a hard
error.

src/mman/mremap.c

+#define _GNU_SOURCE
 #include <unistd.h>
 #include <sys/mman.h>
 #include <errno.h>
@@ -6,19 +7,25 @@
 #include "syscall.h"
 #include "libc.h"
 
+static void dummy(void) { }
+weak_alias(dummy, __vm_wait);
+
 void *__mremap(void *old_addr, size_t old_len, size_t new_len, int flags, ...)
 {
 	va_list ap;
-	void *new_addr;
+	void *new_addr = 0;
 
 	if (new_len >= PTRDIFF_MAX) {
 		errno = ENOMEM;
 		return MAP_FAILED;
 	}
 
-	va_start(ap, flags);
-	new_addr = va_arg(ap, void *);
-	va_end(ap);
+	if (flags & MREMAP_FIXED) {
+		__vm_wait();
+		va_start(ap, flags);
+		new_addr = va_arg(ap, void *);
+		va_end(ap);
+	}
 
 	return (void *)syscall(SYS_mremap, old_addr, old_len, new_len, flags, new_addr);
 }