mount.nfs: restore correct error status when umount fails

mount.nfs: restore correct error status when umount fails

If nfs-utils is built without --enable-libmount-mount, then
an unmount that failed due to the filesystem being busy will
exit with '16' - EX_FILEIO.
Autofs apparently relies on this.

When built with --enable-libmount-mount, the same case will
exit with '32' - EX_FAIL.  Normally this is reserved for
internal errors.

This patch restores the use of EX_FILEIO for errors from umount.

Reviewed-by: Karel Zak <kzak@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>