io_uring/rsrc: always initialize 'folio' to NULL

Smatch complains that:

smatch warnings:
io_uring/rsrc.c:1262 io_sqe_buffer_register() error: uninitialized symbol 'folio'.

'folio' may be used uninitialized, which can happen if we end up with a
single page mapped. Ensure that we clear folio to NULL at the top so
it's always set.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/202302241432.YML1CD5C-lkp@intel.com/


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