fs/aio: Use RCU accessors for kioctx_table->table[]

fs/aio: Use RCU accessors for kioctx_table->table[]

While converting ioctx index from a list to a table, db446a08c23d
("aio: convert the ioctx list to table lookup v3") missed tagging
kioctx_table->table[] as an array of RCU pointers and using the
appropriate RCU accessors.  This introduces a small window in the
lookup path where init and access may race.

Mark kioctx_table->table[] with __rcu and use the approriate RCU
accessors when using the field.

Signed-off-by: Tejun Heo <[email protected]>
Reported-by: Jann Horn <[email protected]>
Fixes: db446a08c23d ("aio: convert the ioctx list to table lookup v3")
Cc: Benjamin LaHaise <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: [email protected] # v3.12+