btrfs: convert end_compressed_writeback() to use filemap_get_folios()

Converted function to use folios throughout.  This is in preparation for
the removal of find_get_pages_contig().  Now also supports large folios.

Since we may receive more than nr_pages pages, nr_pages may underflow. 
Since nr_pages > 0 is equivalent to index <= end_index, we replaced it
with this check instead.

Also this function does not care about the pages being contiguous so we
can just use filemap_get_folios() to be more efficient.

Link: https://lkml.kernel.org/r/20220824004023.77310-4-vishal.moola@gmail.com


Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Acked-by: David Sterba <dsterba@suse.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Chris Mason <clm@fb.com>
Cc: David Sterba <dsterb@suse.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>