MAINTAINERS: add Alex Aring as Reviewer for file locking code

Alex helps co-maintain the DLM code and did some recent work to fix up
how lockd and GFS2 work together. Add him as a Reviewer for file locking
changes.

Acked-by: Alexander Aring <aahringo@redhat.com>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>