BACKPORT: loop: Set correct device size when using LOOP_CONFIGURE

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: 3448914e8cc5("loop: Add LOOP_CONFIGURE ioctl")
Reported-by: Lennart Poettering <mzxreary@0pointer.de>
Tested-by: Yang Xu <xuyang2018.jy@cn.fujitsu.com>
Signed-off-by: Martijn Coenen <maco@android.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 79e5dc59e2974a48764269fa9ff544ae8ffe3338)
Bug: 187129171
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: I823aba7e482eaf347992d507c875c10469a27c16