RDMA/ocrdma: Clarify the mm_head searching

RDMA/ocrdma: Clarify the mm_head searching

The intention of this code is to find matching entries exactly, the driver
never creates phys_addr's with different lens so the current expression is
not a bug, but it doesn't make sense and confuses review tooling.

Search for exact match instead.

Link: https://sashiko.dev/#/patchset/0-v1-e911b76a94d1%2B65d95-rdma_udata_rep_jgg%40nvidia.com?part=4
Link: https://patch.msgid.link/r/8-v1-41f3135e5565+9d2-rdma_ai_fixes1_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>