ocfs2/dlm: Wait on lockres instead of erroring cancel requests

ocfs2/dlm: Wait on lockres instead of erroring cancel requests

In case a downconvert is queued, and a flock receives a signal,
BUG_ON(lockres->l_action != OCFS2_AST_INVALID) is triggered
because a lock cancel triggers a dlmunlock while an AST is
scheduled.

To avoid this, allow a LKM_CANCEL to pass through, and let it
wait on __dlm_wait_on_lockres().

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.de>
Acked-off-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>