iomap: release the folio batch on iomap callback failures

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

iomap: release the folio batch on iomap callback failures

A sashiko review of an unrelated patch points out that the folio
batch mechanism used for iomap zero range fails to release the batch
in a couple error scenarios. If either calls to ->iomap_end() or
->iomap_begin() fail, the direct return paths bypass the batch
cleanup.

The ->iomap_end() case is not a practical issue at the moment
because there is no user of the mechanism that returns an error from
this path. The ->iomap_begin() case is theoretically possible
because XFS can invoke the fill helper and error out at various
points thereafter. This subtly complicates things because XFS does
not transfer iomap_flags to the iomap data structure in the error
path.

To deal with both of these issues, first make sure to invoke the
cleanup helper in the error path for either fs callback. Second,
update the helper to clear the flag unconditionally and release the
batch so long as it is populated. This more clearly delineates the
purpose of the flag to control the I/O path and not necessarily the
status of the fbatch, so add a comment around this as well.