The device size calculation was done before processing the loop
configuration, which meant that the we set the size on the underlying
block device incorrectly in case lo_offset/lo_sizelimit were set in the
configuration. Delay computing the size until we've setup the device
parameters correctly.

Fixes: 3448914e("loop: Add LOOP_CONFIGURE ioctl")
Reported-by: NLennart Poettering <mzxreary@0pointer.de>
Tested-by: NYang Xu <xuyang2018.jy@cn.fujitsu.com>
Signed-off-by: NMartijn Coenen <maco@android.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>