BACKPORT: loop: Fix missing discard support when using LOOP_CONFIGURE

Without calling loop_config_discard() the discard flag and parameters
aren't set/updated for the loop device and worst-case they could
indicate discard support when it isn't the case (ex: if the
LOOP_SET_STATUS ioctl was used with a different file prior to
LOOP_CONFIGURE).

Bug: 254441685
Cc: <stable@vger.kernel.org> # 5.8.x-
Fixes: 3448914e8cc5 ("loop: Add LOOP_CONFIGURE ioctl")
Signed-off-by: Kristian Klausen <kristian@klausen.dk>
Link: https://lore.kernel.org/r/20210618115157.31452-1-kristian@klausen.dk


Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 2b9ac22b12a266eb4fec246a07b504dd4983b16b)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: If36de63cb194fde7470ccf16586ef349a775dc47