aio: fix error handling and rcu usage in "convert the ioctx list to table lookup v3"

In the patch "aio: convert the ioctx list to table lookup v3", incorrect
handling in the ioctx_alloc() error path was introduced that lead to an
ioctx being added via ioctx_add_table() while freed when the ioctx_alloc()
call returned -EAGAIN due to hitting the aio_max_nr limit.  Fix this by
only calling ioctx_add_table() as the last step in ioctx_alloc().

Also, several unnecessary rcu_dereference() calls were added that lead to
RCU warnings where the system was already protected by a spin lock for
accessing mm->ioctx_table.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>