sys/mount.h: add MS_NOSYMFOLLOW from linux v5.10

path resolution does not follow symlinks on nosymfollow mounts (but
readlink still does), see

  linux commit dab741e0e02bd3c4f5e2e97be74b39df2523fc6e
  Add a "nosymfollow" mount option.

include/sys/mount.h

@@ -31,6 +31,7 @@ extern "C" {
 #define MS_REMOUNT     32
 #define MS_MANDLOCK    64
 #define MS_DIRSYNC     128
+#define MS_NOSYMFOLLOW 256
 #define MS_NOATIME     1024
 #define MS_NODIRATIME  2048
 #define MS_BIND        4096