nilfs2: replace WARN_ONs by nilfs_error for checkpoint acquisition failure

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

nilfs2: replace WARN_ONs by nilfs_error for checkpoint acquisition failure

If creation or finalization of a checkpoint fails due to anomalies in the

checkpoint metadata on disk, a kernel warning is generated.

This patch replaces the WARN_ONs by nilfs_error, so that a kernel, booted

with panic_on_warn, does not panic. A nilfs_error is appropriate here to

handle the abnormal filesystem condition.

This also replaces the detected error codes with an I/O error so that

neither of the internal error codes is returned to callers.