fs/adfs: super: fix inode dropping

When we have write support enabled, we must not drop inodes before they
have been written back, otherwise we lose updates to the filesystem on
umount.  Keep the inodes around unless we are built in read-only mode,
or we are mounted read-only.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>