inet: frags: annotate races around fqdir->dead and fqdir->high_thresh

Both fields can be read/written without synchronization,
add proper accessors and documentation.

Fixes: d5dd8879 ("inet: fix various use-after-free in defrags units")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>