fs: dlm: use READ_ONCE for config var

This patch will use READ_ONCE to signal the compiler to read this
variable only one time. If we don't do that it could be that the
compiler read this value more than one time, because some optimizations,
from the configure data which might can be changed during this time.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>