ocfs2: budget for extent tree splits when adding refcount flag

When we're adding the refcount flag to an extent, we have to budget
enough space to handle a full extent btree split in addition to
whatever modifications have to be made to the refcount btree.  We
don't currently do this, with the result that generic/186 crashes
when we need an extent split but not a refcount split because meta_ac
never gets allocated.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/ocfs2/refcounttree.c

@@ -3692,6 +3692,9 @@ int ocfs2_add_refcount_flag(struct inode *inode,
 	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb);
 	struct ocfs2_alloc_context *meta_ac = NULL;
 
+	/* We need to be able to handle at least an extent tree split. */
+	ref_blocks = ocfs2_extend_meta_needed(data_et->et_root_el);
+
 	ret = ocfs2_calc_refcount_meta_credits(inode->i_sb,
 					       ref_ci, ref_root_bh,
 					       p_cluster, num_clusters,