When inserting a new mmap entry to the xarray we should check for
'mmap_page' overflow as it is limited to 32 bits.

Fixes: 40909f66 ("RDMA/efa: Add EFA verbs implementation")
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>