fs: dlm: add comment about lkb IFL flags

This patch adds comments about the difference between the lower 2 bytes
of lkb flags and the 2 upper bytes of the lkb IFL flags. In short the
upper 2 bytes will be handled as internal flags whereas the lower 2
bytes are part of the DLM protocol and are used to exchange messages.

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