rsocket: Fix race in indexer map

Although insertions and removals of rsockets are protected
against accesses to the index map, when reading the map using
a non-rsocket (i.e. normal fd), the reading of the map may
overlap with the removal of an rsocket.  This can result in
accessing freed memory.

Problem reported by: Sasha Kotchubievsky <sashakot@mellanox.com>

Signed-off-by: Sean Hefty <sean.hefty@intel.com>