[1] suggests that fastmap is suitable for large flash devices. Module
parameter 'fm_autoconvert' is a coarse grained switch to enable all
ubi devices to generate fastmap, which may turn on fastmap even for
small flash devices.

This patch imports a new field 'disable_fm' in struct 'ubi_attach_req'
to support following situations by ioctl 'UBI_IOCATT'.
 [old functions]
 A. Disable 'fm_autoconvert': Disbable fastmap for all ubi devices
 B. Enable 'fm_autoconvert': Enable fastmap for all ubi devices
 [new function]
 C. Enable 'fm_autoconvert', set 'disable_fm' for given device: Don't
    create new fastmap and do full scan (existed fastmap will be
    destroyed) for the given ubi device.

A simple test case in [2].

[1] http://www.linux-mtd.infradead.org/doc/ubi.html#L_fastmap
[2] https://bugzilla.kernel.org/show_bug.cgi?id=216278Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>