mm: optimize invalidation of shadow entries

The kernel invalidates the page cache in batches of PAGEVEC_SIZE.  For
each batch, it traverses the page cache tree and collects the entries
(folio and shadow entries) in the struct folio_batch.  For the shadow
entries present in the folio_batch, it has to traverse the page cache tree
for each individual entry to remove them.  This patch optimize this by
removing them in a single tree traversal.

To evaluate the changes, we created 200GiB file on a fuse fs and in a
memcg.  We created the shadow entries by triggering reclaim through
memory.reclaim in that specific memcg and measure the simple
fadvise(DONTNEED) operation.

 # time xfs_io -c 'fadvise -d 0 ${file_size}' file

              time (sec)
Without       5.12 +- 0.061
With-patch    4.19 +- 0.086 (18.16% decrease)

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Shakeel Butt <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Omar Sandoval <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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