bonding: Don't destroy bonding master when removing slave via sysfs

It is wrong to destroy a bonding master from a context that uses the sysfs
of that bond. When last IPoIB slave is unenslaved from by writing to a
sysfs file (for bond0 this would be /sys/class/net/bond0/bonding/slaves)
the driver tries to destroy the bond. This is wrong and can lead to a
lockup or a crash.  This fix lets the bonding master stay and relies on
the user to destroy the bonding master if necessary (i.e. before module
ib_ipoib is unloaded)

This patch affects only bonds of IPoIB slaves. Ethernet slaves stay
unaffected.

Signed-off-by: Moni Shoua <monis@voltaire.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>