ocfs2: fix missing metadata reservation for large xattrs

Summary

In the Linux kernel, the following vulnerability has been resolved: ocfs2: fix missing metadata reservation for large xattrs [BUG] lsetxattr() panics the kernel when setting a large xattr value on a fragmented filesystem where the file already has an external xattr block. [CAUSE] ocfs2_calc_xattr_set_need() never reserves metadata blocks for a new xattr value's extent tree when the file already has an external xattr block. The not_found path leaves meta_add at zero, so meta_ac is NULL when ocfs2_xattr_extend_allocation() runs. A new value root has room for a single extent record. On a fragmented filesystem, the allocator cannot satisfy the xattr value in one contiguous run, so each non-contiguous run requires its own extent record. When the value root's extent list is full and meta_ac is NULL, ocfs2_add_clusters_in_btree() returns RESTART_META, and ocfs2_xattr_extend_allocation() hits BUG_ON(why == RESTART_META). [FIX] The case where no xattr block exists yet already calls ocfs2_extend_meta_needed(&def_xv.xv.xr_list) to reserve value tree metadata. Add the same reservation to the case where an xattr block already exists, making the two cases consistent. Replace the BUG_ON with a -ENOSPC return so that if RESTART_META is returned despite the reservation, the error propagates to userspace instead of panicking the kernel.