gfs2: qd_check_sync cleanups

Rename qd_check_sync() to qd_grab_sync() and make it return a bool.
Turn the sync_gen pointer into a regular u64 and pass in U64_MAX instead
of a NULL pointer when sync generation checking isn't needed.

Introduce a new qd_ungrab_sync() helper for undoing the effects of
qd_grab_sync() if the subsequent bh_get() on the qd object fails.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>