eth: fbnic: avoid double free when failing to DMA-map FW msg

eth: fbnic: avoid double free when failing to DMA-map FW msg

The semantics are that caller of fbnic_mbx_map_msg() retains
the ownership of the message on error. All existing callers
dutifully free the page.

Fixes: da3cde08209e ("eth: fbnic: Add FW communication mechanism")
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250616195510.225819-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>