The variable up.resv is not initialized and is being checking for a
non-zero value in the call to _io_register_rsrc_update. Fix this by
explicitly setting the variable to 0.

Addresses-Coverity: ("Uninitialized scalar variable)"
Fixes: c3bdad02 ("io_uring: add generic rsrc update with tags")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210426094735.8320-1-colin.king@canonical.comSigned-off-by: NJens Axboe <axboe@kernel.dk>