uid_sys_stats: reduce update_io_stats overhead

Replaced read_lock with rcu_read_lock to reduce time that preemption
is disabled.

Added a function to update io stats for specific uid and moved
hash table lookup, user_namespace out of loops.

Bug: 37319300
Change-Id: I2b81b5cd3b6399b40d08c3c14b42cad044556970
Signed-off-by: Jin Qian <jinqian@google.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>