btrfs: switch compression callbacks to direct calls

The indirect calls bring some overhead due to spectre vulnerability
mitigations. The number of cases is small and below the threshold
(10-20) where indirect call would be better.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/zlib.c

@@ -420,9 +420,6 @@ const struct btrfs_compress_op btrfs_zlib_compress = {
 	.put_workspace		= zlib_put_workspace,
 	.alloc_workspace	= zlib_alloc_workspace,
 	.free_workspace		= zlib_free_workspace,
-	.compress_pages		= zlib_compress_pages,
-	.decompress_bio		= zlib_decompress_bio,
-	.decompress		= zlib_decompress,
 	.max_level		= 9,
 	.default_level		= BTRFS_ZLIB_DEFAULT_LEVEL,
 };