RDMA/bnxt_re: Enhance dpi lifecycle logic in doorbell uapis

RDMA/bnxt_re: Enhance dpi lifecycle logic in doorbell uapis

If the DPI is freed when the dbr object is freed, but if the
process has not unmapped the page yet, then the DPI slot could
get reallocated to another process while the original process
still has it mapped. To prevent this, save the DPI info in the
mmap entry during dbr allocation and free the DPI slot from
bnxt_re_mmap_free(), which enures that there are no references
to it.

This change is needed to support doorbell allocation to QPs
in the next patch.

Link: https://patch.msgid.link/r/20260519150041.7251-8-sriharsha.basavapatna@broadcom.com
Signed-off-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com>
Reviewed-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>