fs/adfs: dir: add common directory sync method

adfs_fplus_sync() can be used for both directory formats since we now
have a common way to access the buffer heads, so move it into dir.c
and appropriately rename it.  Remove the directory-format specific
implementations.

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