UBI: block: Fix block device size setting

We are currently taking the block device size from the ubi_volume_info.size
field. However, this is not the amount of data in the volume, but the
number of reserved physical eraseblocks, and hence leads to an incorrect
representation of the volume.

In particular, this produces I/O errors on static volumes as the block
interface may attempt to read unmapped PEBs:

$ cat /dev/ubiblock0_0 > /dev/null
UBI error: ubiblock_read_to_buf: ubiblock0_0 ubi_read error -22
end_request: I/O error, dev ubiblock0_0, sector 9536
Buffer I/O error on device ubiblock0_0, logical block 2384
[snip]

Fix this by using the ubi_volume_info.used_bytes field which is set to the
actual number of data bytes for both static and dynamic volumes.

While here, improve the error message to be less stupid and more useful:
UBI error: ubiblock_read_to_buf: ubiblock0_1 ubi_read error -9 on LEB=0, off=15872, len=512

It's worth noticing that the 512-byte sector representation of the volume
is only correct if the volume size is multiple of 512-bytes. This is true for
virtually any NAND device, given eraseblocks and pages are 512-byte multiple
and hence so is the LEB size.

Artem: tweak the error message and make it look more like other UBI error
messages.

Fixes: 9d54c8a3 ("UBI: R/O block driver on top of UBI volumes")
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: stable@vger.kernel.org # v3.15+