autofs4: Modify autofs_wait to use current_uid() and current_gid()

The code used to do that and then I mucked with it and never quite put
the code back.  Today the code references current_cred()->uid and
current_cred()->gid which is equivalent but more wordy, and not
idiomatic.

Fixes: 93faccbb ("fs: Better permission checking for submounts")
Fixes: 069d5ac9 ("autofs:  Fix automounts by using current_real_cred()->uid")
Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>