io_uring: fix unchecked error in switch_start()

io_rsrc_node_switch_start() can fail, don't forget to check returned
error code.

Reported-by:  <syzbot+a4715dd4b7c866136f79@syzkaller.appspotmail.com>
Fixes: eae071c9

 ("io_uring: prepare fixed rw for dynanic buffers")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c4c06e2f3f0c8e43bd8d0a266c79055bcc6b6e60.1619693112.git.asml.silence@gmail.com


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