ANDROID: Add EXPORT_SYMBOL_GPL for folio_mapcount

We need to get the number of folio mappings through folio_mapcount. Later, pages with mapcount higher than a certain threshold will be skipped for reverse mapping to reduce the high load caused by
reverse mapping during the recycling process.

Bug: 407725951
Bug: 407900088
Bug: 404067677

Change-Id: I21dd847a07fb4e7bb616a3bc01b7d1cdf46e9b0b
Signed-off-by: Marcus Ma <maminghui5@xiaomi.corp-partner.google.com>