scsi: lpfc: refactor mailbox structure context fields

The driver data structure for managing a mailbox command contained two
context fields. Unfortunately, the context were considered "generic" to be
used at the whim of the command code.  Of course, one section of code used
fields this way, while another did it that way, and eventually there were
mixups.

Refactored the structure so that the generic contexts become a node context
and a buffer context and all code standardizes on their use.

Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>