btrfs: compression: inline free_workspace

Replace indirect calls to free_workspace by switch and calls to the
specific callbacks. This is mainly to get rid of the indirection due to
spectre vulnerability mitigations.

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

@@ -400,7 +400,6 @@ next:
 
 const struct btrfs_compress_op btrfs_zlib_compress = {
 	.workspace_manager	= &wsm,
-	.free_workspace		= zlib_free_workspace,
 	.max_level		= 9,
 	.default_level		= BTRFS_ZLIB_DEFAULT_LEVEL,
 };