lmLogOpen() broken failure exit

Callers of lmLogOpen() expect it to return -E... on failure exits, which
is what it returns, except for the case of blkdev_get_by_dev() failure.
It that case lmLogOpen() return the error with the wrong sign...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Dave Kleikamp <dave.kleikamp@oracle.com>