linux-user: Add support for btrfs ioctls used to scrub a filesystem

linux-user: Add support for btrfs ioctls used to scrub a filesystem

This patch implements functionality for following ioctls:

BTRFS_IOC_SCRUB - Starting a btrfs filesystem scrub

    Start a btrfs filesystem scrub. The third ioctls argument
    is a pointer to a following type:

    struct btrfs_ioctl_scrub_args {
	__u64 devid;				/* in */
	__u64 start;				/* in */
	__u64 end;				/* in */
	__u64 flags;				/* in */
	struct btrfs_scrub_progress progress;	/* out */
	/* pad to 1k */
	__u64 unused[(1024-32-sizeof(struct btrfs_scrub_progress))/8];
    };

    Before calling this ioctl, field 'devid' should be filled
    with value that represents the device id of the btrfs filesystem
    for which the scrub is to be started.

BTRFS_IOC_SCRUB_CANCEL - Canceling scrub of a btrfs filesystem

    Cancel a btrfs filesystem scrub if it is running. The third
    ioctls argument is ignored.

BTRFS_IOC_SCRUB_PROGRESS - Getting status of a running scrub

    Read the status of a running btrfs filesystem scrub. The third
    ioctls argument is a pointer to the above mentioned
    'struct btrfs_ioctl_scrub_args'. Similarly as with 'BTRFS_IOC_SCRUB',
    the 'devid' field should be filled with value that represents the
    id of the btrfs device for which the scrub has started. The status
    of a running scrub is returned in the field 'progress' which is
    of type 'struct btrfs_scrub_progress' and its definition can be
    found at:
    https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/btrfs.h#L150

Implementation nots:

    Ioctls in this patch use type 'struct btrfs_ioctl_scrub_args' as their
    third argument. That is the reason why an aproppriate thunk type
    definition is added in file 'syscall_types.h'.

Signed-off-by: Filip Bozuta <Filip.Bozuta@syrmia.com>
Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200823195014.116226-9-Filip.Bozuta@syrmia.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>