kgsl: hold rcu lock around find_task_by_vpid()

The task list must be rcu-protected.

Fixes:
    ===============================
    [ INFO: suspicious RCU usage. ]
    3.4.0-g4a73a1d-00004-gf7d262d-dirty #1 Tainted: G        W  O
    -------------------------------
    /home/bob/ext/cmbuild/kernel/sony/apq8064/kernel/pid.c:425 find_task_by_pid_ns() needs rcu_read_lock() protection!

    other info that might help us debug this:

    rcu_scheduler_active = 1, debug_locks = 1
    3 locks held by kworker/u:2/106:
     #0:  ((device->name)){.+.+.+}, at: [<c00b12f4>] process_one_work+0x1d8/0x698
     #1:  ((device_3d0.dev).hang_check_ws){+.+.+.}, at: [<c00b12f4>] process_one_work+0x1d8/0x698
     #2:  ((device_3d0.dev).mutex){+.+.+.}, at: [<c04163ac>] kgsl_hang_check+0x2c/0x84

    stack backtrace:
    [<c0015ec4>] (unwind_backtrace+0x0/0x128) from [<c08c31dc>] (dump_stack+0x20/0x24)
    [<c08c31dc>] (dump_stack+0x20/0x24) from [<c00e5094>] (lockdep_rcu_suspicious+0xd0/0x10c)
    [<c00e5094>] (lockdep_rcu_suspicious+0xd0/0x10c) from [<c00b318c>] (find_task_by_pid_ns+0x88/0xb0)
    [<c00b318c>] (find_task_by_pid_ns+0x88/0xb0) from [<c00b31e4>] (find_task_by_vpid+0x30/0x34)
    [<c00b31e4>] (find_task_by_vpid+0x30/0x34) from [<c0426a2c>] (snapshot_os+0x1a8/0x228)
    [<c0426a2c>] (snapshot_os+0x1a8/0x228) from [<c0426bc8>] (kgsl_device_snapshot+0x11c/0x26c)
    [<c0426bc8>] (kgsl_device_snapshot+0x11c/0x26c) from [<c043bc40>] (adreno_dump_and_exec_ft+0xbc/0xae0)
    [<c043bc40>] (adreno_dump_and_exec_ft+0xbc/0xae0) from [<c04163d4>] (kgsl_hang_check+0x54/0x84)
    [<c04163d4>] (kgsl_hang_check+0x54/0x84) from [<c00b1458>] (process_one_work+0x33c/0x698)
    [<c00b1458>] (process_one_work+0x33c/0x698) from [<c00b1b34>] (worker_thread+0x1a8/0x2cc)
    [<c00b1b34>] (worker_thread+0x1a8/0x2cc) from [<c00b6a10>] (kthread+0xa8/0xbc)
    [<c00b6a10>] (kthread+0xa8/0xbc) from [<c000f7bc>] (kernel_thread_exit+0x0/0x8)

Signed-off-by: Bob Copeland <me@bobcopeland.com>