The bg option is essentially ignored with nfs4 currently. nfs4mount()

will never exit with EX_BG, so the mount will never be backgrounded.
Fix it so that when bg is specified that we error out with EX_BG as
soon as possible after the first failed mount attempt.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>