I would recommend that you not introduce a new MNT_GJOURNAL flag to
and that instead you just pass -o gjournal directly down into
nmount(). In kernel code, you can use vfs_flagopt()/vfs_getopt()
to determine if you have this mount option or not.
The mount(8) userland utility would not need any modifications,
since it just passes -o options down to nmount().