Torn page hazard in ginRedoUpdateMetapage()

When GIN changes a metapage, we WAL-log its ex-header content and never use a
backup block.  This reduces WAL volume since the vast majority of the metapage
is unused.  However, ginRedoUpdateMetapage() only restores the WAL-logged
content if the metapage LSN predates the WAL record LSN.  If a metapage write
tore and updated the LSN but not the other content, we would fail to complete
the update.  Instead, unconditionally reinitialize the metapage similar to how
_bt_restore_meta() handles the situation.

I found this problem by code reading and did not attempt to build a test case
illustrating its practical consequences.  It's possible that there's no
problem in practice on account of some reason I haven't contemplated.

Thanks,
nm