xfs: Use WARN_ON_ONCE for bailout mount-operation

If the CONFIG_BUG is enabled, BUG is executed and then system is crashed.
However, the bailout for mount is no longer proceeding.

Using WARN_ON_ONCE rather than BUG can prevent this situation.
Signed-off-by: Austin Kim <austindh.kim@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_mount.c

@@ -214,7 +214,7 @@ xfs_initialize_perag(
 
 		spin_lock(&mp->m_perag_lock);
 		if (radix_tree_insert(&mp->m_perag_tree, index, pag)) {
-			BUG();
+			WARN_ON_ONCE(1);
 			spin_unlock(&mp->m_perag_lock);
 			radix_tree_preload_end();
 			error = -EEXIST;