dm mpath: Interface for explicit probing of active paths

Multipath cannot directly provide failover for ioctls in the kernel
because it doesn't know what each ioctl means and which result could
indicate a path error. Userspace generally knows what the ioctl it
issued means and if it might be a path error, but neither does it know
which path the ioctl took nor does it necessarily have the privileges to
fail a path using the control device.

In order to allow userspace to address this situation, implement a
DM_MPATH_PROBE_PATHS ioctl that prompts the dm-mpath driver to probe all
active paths in the current path group to see whether they still work,
and fail them if not. If this returns success, userspace can retry the
ioctl and expect that the previously hit bad path is now failed (or
working again).

The immediate motivation for this is the use of SG_IO in QEMU for SCSI
passthrough. Following a failed SG_IO ioctl, QEMU will trigger probing
to ensure that all active paths are actually alive, so that retrying
SG_IO at least has a lower chance of failing due to a path error.
However, the problem is broader than just SG_IO (it affects any ioctl),
and if applications need failover support for other ioctls, the same
probing can be used.

This is not implemented on the DM control device, but on the DM mpath
block devices, to allow all users who have access to such a block device
to make use of this interface, specifically to implement failover for
ioctls. For the same reason, it is also unprivileged. Its implementation
is effectively just a bunch of reads, which could already be issued by
userspace, just without any guarantee that all the rights paths are
selected.

The probing implemented here is done fully synchronously path by path;
probing all paths concurrently is left as an improvement for the future.

Co-developed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

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