aio: restore locking of ioctx list on removal

Commit 36f55889
"aio: refcounting cleanup" resulted in ioctx_lock not being held
during ctx removal, leaving the list susceptible to corruptions.

In mainline kernel the issue went away as a side effect of
db446a08c23d5475e6b08c87acca79ebb20f283c "aio: convert the ioctx list to
table lookup v3".

Fix the problem by restoring appropriate locking.

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Reported-by: Eryu Guan <eguan@redhat.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Kent Overstreet <kmo@daterainc.com>
Acked-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>