rpc.mountd: fix memory leak and error handling in nfsd_fh

rpc.mountd: fix memory leak and error handling in nfsd_fh

nfsd_fh() uses strdup for creating found_path and doesn't check the
return value. It also doesn't free this memory when the function
returns. Check the return value of strdup and return immediately
if it's NULL. Also, free found_path on exit.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Neil Brown <neilb@suse.de>