umount.nfs: umount doesn't recognize a busy file system

umount.nfs shouldn't remove a busy file system from /etc/mtab, and should
report and return an error.  I also added an extra "goto" to make the flow
of control more clear, and to reduce the chance that a future change in
this logic will break it.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>