fs/adfs: dir: improve compiler coverage in adfs_dir_update

Get rid of the ifdef, using IS_ENABLED() instead to detect whether the
code should be callable.  This allows the compiler to always parse the
following code, reducing the chances of errors being missed.

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