ubi: fastmap: Add fastmap control support for 'UBI_IOCATT' ioctl

[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=216278

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

include/uapi/mtd/ubi-user.h

@@ -247,6 +247,7 @@ enum {
  * @vid_hdr_offset: VID header offset (use defaults if %0)
  * @max_beb_per1024: maximum expected number of bad PEB per 1024 PEBs
  * @padding: reserved for future, not used, has to be zeroed
+ * @disable_fm: whether disable fastmap
  *
  * This data structure is used to specify MTD device UBI has to attach and the
  * parameters it has to use. The number which should be assigned to the new UBI
@@ -281,13 +282,18 @@ enum {
  * eraseblocks for new bad eraseblocks, but attempts to use available
  * eraseblocks (if any). The accepted range is 0-768. If 0 is given, the
  * default kernel value of %CONFIG_MTD_UBI_BEB_LIMIT will be used.
+ *
+ * If @disable_fm is not zero, ubi doesn't create new fastmap even the module
+ * param 'fm_autoconvert' is set, and existed old fastmap will be destroyed
+ * after doing full scanning.
  */
 struct ubi_attach_req {
 	__s32 ubi_num;
 	__s32 mtd_num;
 	__s32 vid_hdr_offset;
 	__s16 max_beb_per1024;
-	__s8 padding[10];
+	__s8 disable_fm;
+	__s8 padding[9];
 };
 
 /*