net/af_xdp: fix shared UMEM fill queue reserve

net/af_xdp: fix shared UMEM fill queue reserve

Commit 81fe6720f84f ("net/af_xdp: reserve fill queue before socket create")
moves the fill queue reserve logic to before the creation of the socket in
order to suppress kernel logs like:

XSK buffer pool does not provide enough addresses to fill 2047 buffers on
Rx ring 0

However, for queues that share umem, the fill queue reserve must occur
after the socket creation, because the fill queue is not valid until
that point.

This commit uses the umem refcnt value to determine whether the queue is
sharing a umem, and performs the fill queue reservation either before or
after the socket creation, depending on the refcnt value.

The kernel logs will still be seen for the shared umem queues.

Fixes: 81fe6720f84f ("net/af_xdp: reserve fill queue before socket create")
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>