scsi: lpfc: Define types in a union for generic void *context3 ptr

scsi: lpfc: Define types in a union for generic void *context3 ptr

In LPFC_MBOXQ_t, the void *context3 ptr is used for various paths.  It is
treated as a generic pointer, and is type casted during its usage.

The issue with this is that it can sometimes get confusing when reading
code as to what the context3 ptr is being used for and mistakenly be reused
in a different context.

Rename context3 to ctx_u, and declare it as a union of defined ptr types.
From now on, the ctx_u ptr may be used only if users define the use case
type.

Signed-off-by: Justin Tee <justin.tee@broadcom.com>
Link: https://lore.kernel.org/r/20240305200503.57317-11-justintee8345@gmail.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>