datapath: fix sparse warning in function tbl_mask_array_delete_mask()

Sparse gives "incompatible types in comparison expression (different
address spaces)" warning messages. Fix this by add rcu_dereference()
wrappers.

Reported-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>