ubiattach: introduce disable_fm in UBI_IOCATT

ubiattach: introduce disable_fm in UBI_IOCATT

The ioctl UBI_IOCATT has been extended with disable_fm parameter after
[1].

This parameter is used for disabling fastmap for target ubi device.
If 'disable_fm' is set, ubi doesn't create new fastmap even the module
param 'fm_autoconvert' is set, and existed old fastmap will be destroyed
after attaching process.

A simple test case in [2].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=669d204469c46e91d99da24914130f78277a71d3
[2] https://bugzilla.kernel.org/show_bug.cgi?id=216278

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>