generic/459: improve shutdown/read-only check to accommodate bcachefs

generic/459: improve shutdown/read-only check to accommodate bcachefs

generic/459 occasionally fails on bcachefs because the deliberately
induced I/O errors caused by exhausting the overprovisioned thin
pool can lead to filesystem shutdown. This test considers this
expected behavior on certain fs', but only checks for the ext4
remount read-only behavior. bcachefs does a similar emergency
read-only transition in response to certain I/O errors, but it
behaves more similar to an XFS shutdown and doesn't necessarily
reflect "ro" state in the mount table (unless induced by userspace).

Since the test already runs a touch command to help trigger the ext4
error handling sequence, this can be tweaked to serve double duty
and also more accurately detect read-only status on bcachefs.
Refactor into a small helper, check for touch command failure, and
consider the fs read-only if either that or the mount entry check
indicates it.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>