ibmvnic: define map_rxpool_buf_to_ltb()

Define a helper to map a given rx pool buffer into its corresponding long
term buffer (LTB) and offset. Currently there is just one LTB per pool so
the mapping is trivial. When we add support for multiple LTBs per pool,
this helper will be more useful.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>