RDMA/qedr: Fix null-pointer dereference when calling rdma_user_mmap_get_offset

RDMA/qedr: Fix null-pointer dereference when calling rdma_user_mmap_get_offset

When running against rdma-core that doesn't support doorbell recovery, the
rdma_user_mmap_entry won't be allocated for doorbell recovery related
mappings.

We have a flag indicating whether rdma-core supports doorbell recovery or
not which was used during initialization, however some cases didn't check
that the rdma_user_mmap_entry exists before attempting to acquire it's
offset.

Fixes: 7bf629501d66 ("RDMA/qedr: Add doorbell overflow recovery support")
Link: https://lore.kernel.org/r/20191118150645.26602-1-michal.kalderon@marvell.com
Signed-off-by: Ariel Elior <ariel.elior@marvell.com>
Signed-off-by: Michal Kalderon <michal.kalderon@marvell.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>