linux-user: Add support for btrfs ioctls used to get/set features

linux-user: Add support for btrfs ioctls used to get/set features

This patch implements functionality for following ioctls:

BTRFS_IOC_GET_FEATURES - Getting feature flags

    Read feature flags for a btrfs filesystem. The feature flags
    are returned inside the ioctl's third argument which represents
    a pointer to a following structure type:

    struct btrfs_ioctl_feature_flags {
	__u64 compat_flags;
	__u64 compat_ro_flags;
	__u64 incompat_flags;
    };

    All of the structure field represent bit masks that can be composed
    of values which can be found on:
    https://elixir.bootlin.com/linux/latest/source/fs/btrfs/ctree.h#L282

BTRFS_IOC_SET_FEATURES - Setting feature flags

    Set and clear feature flags for a btrfs filesystem. The feature flags
    are set using the ioctl's third argument which represents a
    'struct btrfs_ioctl_feature_flags[2]' array. The first element of the
    array represent flags which are to be cleared and the second element of
    the array represent flags which are to be set. The second element has the
    priority over the first, which means that if there are matching flags
    in the elements, they will be set in the filesystem. If the flag values
    in the third argument aren't correctly set to be composed of the available
    predefined flag values, errno ENOPERM ("Operation not permitted") is returned.

BTRFS_IOC_GET_SUPPORTED_FEATURES - Getting supported feature flags

    Read supported feature flags for a btrfs filesystem. The supported
    feature flags are read using the ioctl's third argument which represents
    a 'struct btrfs_ioctl_feature_flags[3]' array. The first element of this
    array represents all of the supported flags in the btrfs filesystem.
    The second element represents flags that can be safely set and third element
    represent flags that can be safely clearead.

Implementation notes:

    All of the implemented ioctls use 'struct btrfs_ioctl_feature_flags' as
    third argument. That is the reason why a corresponding defintion was added
    in file 'linux-user/syscall_types.h'.

Signed-off-by: Filip Bozuta <Filip.Bozuta@syrmia.com>
Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200823195014.116226-5-Filip.Bozuta@syrmia.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>