net: mana: hardening: Validate doorbell ID from GDMA_REGISTER_DEVICE response

As a part of MANA hardening for CVM, add validation for the doorbell
ID (db_id) received from hardware in the GDMA_REGISTER_DEVICE response
to prevent out-of-bounds memory access when calculating the doorbell
page address.

In mana_gd_ring_doorbell(), the doorbell page address is calculated as:
  addr = db_page_base + db_page_size * db_index
       = (bar0_va + db_page_off) + db_page_size * db_index

A hardware could return values that cause this address to fall outside
the BAR0 MMIO region. In Confidential VM environments, hardware responses
cannot be fully trusted.

Add the following validations:
- Store the BAR0 size (bar0_size) in gdma_context during probe.
- Validate the doorbell page offset (db_page_off) read from device
  registers does not exceed bar0_size during initialization, converting
  mana_gd_init_registers() to return an error code.
- Validate db_id from GDMA_REGISTER_DEVICE response against the
  maximum number of doorbell pages that fit within BAR0.

Signed-off-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
Link: https://patch.msgid.link/20260306211212.543376-1-ernis@linux.microsoft.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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