rs-preload: Handle recursive socket() calls

When ACM support is enabled in the librdmacm, it will attempt to
establish a socket connection to the ACM daemon.  When the rsocket
preload library is in use, this can result in a recursive call
to socket() that results in the library hanging.  The resulting
call stack is:

socket() -> rsocket() -> rdma_create_id() -> ucma_init() ->
socket() -> rsocket() -> rdma_create_id() -> ucma_init()

The second call to ucma_init() hangs because initialization is
still pending.

Fix this by checking for recursive calls to socket() in the preload
library.  When detected, call the real socket() call instead of
directing the call back into rsockets().  Since rsockets is a part
of the librdmacm, it can call rsockets directly if it wants to use
rsockets instead of standard sockets.

This problem and the cause was reported by Chet Murthy <chet@watson.ibm.com>

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