additional -r (read-only) flag or or -w (read-write) flag,
then assume we want, mount -u -w.
When doing a mount update, this will implicitly pass a "noro" mount
option down to the VFS layer.
vfs_mergeopts() in vfs_mount.c will then remove the "ro" mount option
if it exists in the mount options for a mounted file system.
This means that "mount -u" works the same as "mount -u -w"
and will convert a read-only mount to read-write.
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