erofs: fix missing xas_retry() in fscache mode

In the Linux kernel, the following vulnerability has been resolved:

erofs: fix missing xas_retry() in fscache mode

The xarray iteration only holds the RCU read lock and thus may encounter

XA_RETRY_ENTRY if there's process modifying the xarray concurrently.

This will cause oops when referring to the invalid entry.

Fix this by adding the missing xas_retry(), which will make the

iteration wind back to the root node if XA_RETRY_ENTRY is encountered.