btrfs: invalidate pages instead of truncate after reflinking

btrfs: invalidate pages instead of truncate after reflinking

Qu reported that generic/164 often fails because the read operations get
zeroes when it expects to either get all bytes with a value of 0x61 or
0x62. The issue stems from truncating the pages from the page cache
instead of invalidating, as truncating can zero page contents. This
zeroing is not just in case the range is not page sized (as it's commented
in truncate_inode_pages_range()) but also in case we are using large
folios, they need to be split and the splitting fails. Stealing Qu's
comment in the thread linked below:

  "We can have the following case:

	0          4K         8K         12K          16K
        |          |          |          |            |
        |<---- Extent A ----->|<----- Extent B ------>|

   The page size is still 4K, but the folio we got is 16K.

   Then if we remap the range for [8K, 16K), then
   truncate_inode_pages_range() will get the large folio 0 sized 16K,
   then call truncate_inode_partial_folio().

   Which later calls folio_zero_range() for the [8K, 16K) range first,
   then tries to split the folio into smaller ones to properly drop them
   from the cache.

   But if splitting failed (e.g. racing with other operations holding the
   filemap lock), the partially zeroed large folio will be kept, resulting
   the range [8K, 16K) being zeroed meanwhile the folio is still a 16K
   sized large one."

So instead of truncating, invalidate the page cache range with a call to
filemap_invalidate_inode(), which besides not doing any zeroing also
ensures that while it's invalidating folios, no new folios are added.

This helps ensure that buffered reads that happen while a reflink
operation is in progress always get either the whole old data (the one
before the reflink) or the whole new data, which is what generic/164
expects.

Link: https://lore.kernel.org/linux-btrfs/7fb9b44f-9680-4c22-a47f-6648cb109ddf@suse.com/
Reported-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>