ibmvnic: Reuse LTB when possible

Reuse the long term buffer during a reset as long as its size has
not changed. If the size has changed, free it and allocate a new
one of the appropriate size.

When we do this, alloc_long_term_buff() and reset_long_term_buff()
become identical. Drop reset_long_term_buff().

Reviewed-by: Rick Lindsley <ricklind@linux.vnet.ibm.com>
Reviewed-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>