ANDROID: binder: fix sleeping from invalid function caused by RT inheritance

When changing a thread's scheduling priority, binder calls
sched_setscheduler_nocheck() while holding the node lock and
proc inner lock. This was safe until v5.3 when a change was
introduced where cpuset_read_lock() is called in this path
which can sleep: commit 710da3c8 ("sched/core: Prevent
race condition between cpuset and __sched_setscheduler()").

Refactored binder_proc_transaction() to avoid holding a lock
when calling binder_transaction_priority().

Bug: 143627611
Change-Id: I405c76b4813777905090ccc33e4f048b37700068
Fixes: e00eb41c ("ANDROID: binder: add support for RT prio inheritance.")
Signed-off-by: Todd Kjos <tkjos@google.com>