bpf: Fix race condition with map pinning

bpf: Fix race condition with map pinning

If two processes attempt to invoke bpf_map_attach() at the same time,
then they will both create maps, then the first will successfully pin
the map to the filesystem and the second will not pin the map, but will
continue operating with a reference to its own copy of the map. As a
result, the sharing of the same map will be broken from the two programs
that were concurrently loaded via loaders using this library.

Fix this by adding a retry in the case where the pinning fails because
the map already exists on the filesystem. In that case, re-attempt
opening a fd to the map on the filesystem as it shows that another
program already created and pinned a map at that location.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>