doc/OBD-HOWTO.sgml: updated references to attach types from *_obd to obd*

doc/OBD-HOWTO.sgml: updated references to attach types from *_obd to obd*
    and also mentioned autoloading of class modules.
doc/obdspec.sgml: added paragraph on usage of o_valid when passing between
    class drivers, to ensure consistency of metadata in each driver.
    Updated punch definition.
ext2obd/ext2_obd.c: ensure that this driver only returns o_valid flags for
    fields which it has set
obdfs/flushd.c: ensure all pages are flushed at unmount time
obdfs/super.c: fix fast symlinks (from 2.3.42)
snap/snap.c: ensure we keep o_valid safe across method calls if required.
    Removed hacks which are resolved by proper o_valid handling in ext2obd.