dlm: section mismatch warning fix

Removed the section mismatch message:
WARNING: fs/dlm/dlm.o(.init.text+0x132): Section mismatch in reference from the function init_module() to the function .exit.text:dlm_netlink_exit()

Since dlm_netlink_exit() is called in the init_dlm() error handling,
the __exit annotation has been removed.

Signed-off-by: Leonardo Potenza <lpotenza@inwind.it>
Signed-off-by: David Teigland <teigland@redhat.com>