pidfd: Use new infrastructure to fix deadlocks in execve

This changes __pidfd_fget to use the new exec_update_mutex
instead of cred_guard_mutex.

This should be safe, as the credentials do not change
before exec_update_mutex is locked.  Therefore whatever
file access is possible with holding the cred_guard_mutex
here is also possbile with the exec_update_mutex.

Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>