btrfs: Drop __exit attribute on btrfs_exit_compress

As this function is called in some error paths while not removing the module, the __exit attribute prevents the kernel image from linking when btrfs is compiled in statically.

---
I am not completely sure whether this has already been reported, but googling around did not show any traces.

Best regards,
Alexey

index f745287..db36b5e 100644
@@ -900,7 +900,7 @@ int btrfs_decompress(int type, unsigned char *data_in, struct page *dest_page,
 	return ret;
 }
 
-void __exit btrfs_exit_compress(void)
+void btrfs_exit_compress(void)
 {
 	free_workspaces();
 }