autofs: rename sx lock description for am_lock

Rename am_lock description from autofslk -> autfsm.

The lock description, autofslk, is used as the description for
autofs_softc->sc_lock, which is used to protect autofs requests and the
like as opposed to am_lock which protects autofs nodes for a given
mount.

This change allows witness to distinguish different lock orders for each
lock.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D57972