mm/filemap: reinitialize folio->_mapcount directly

mm/filemap: reinitialize folio->_mapcount directly

Let's get rid of the page_mapcount_reset() call and simply reinitialize
folio->_mapcount directly.

Link: https://lkml.kernel.org/r/20240529111904.2069608-6-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>	[zram/zsmalloc workloads]
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>