fs: dlm: handle sequence numbers as atomic

Currently seq_next is only be read on the receive side which processed
in an ordered way. The seq_send is being protected by locks. To being
able to read the seq_next value on send side as well we convert it to an
atomic_t value. The atomic_cmpxchg() is probably not necessary, however
the atomic_inc() depends on a if coniditional and this should be handled
in an atomic context.

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