Avoid a potential race when task exec's and we get a new ->mm but check against
the old credentials in ptrace_may_access().

Holding of the mutex is implemented by factoring out the body of the code into a
helper function __check_mem_permission().  Performing this factorization now
simplifies upcoming changes and minimizes churn in the diff's.
Signed-off-by: NStephen Wilson <wilsons@start.ca>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>