Patch from Manfred Spraul <manfred@colorfullife.com>

exec of setuid apps and ptrace must be synchronized, to ensure that a normal
user cannot ptrace a setuid app across exec.  ptrace_attach acquires the
task_lock around the uid checks, compute_creds acquires the BLK.  The patch
converts compute_creds to the task_lock.  Additionally, it removes the
do_unlock variable: the task_lock is not heaviliy used, there is no need to
avoid the spinlock by adding branches.

The patch is a cleanup patch, not a fix for a security problem: AFAICS the
sys_ptrace in every arch acquires the BKL before calling ptrace_attach.