io_uring: fix io_recvmsg_prep_multishot sparse warnings

Fix casts missing the __user parts. This seemed to only cause errors on
the alpha build, or if checked with sparse, but it was definitely an
oversight.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 9bb66906 ("io_uring: support multishot in recvmsg")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220805115450.3921352-1-dylany@fb.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>