RDMA/uverbs: Introduce UCAP (User CAPabilities) API

Implement a new User CAPabilities (UCAP) API to provide fine-grained
control over specific firmware features.

This approach offers more granular capabilities than the existing Linux
capabilities, which may be too generic for certain FW features.

This mechanism represents each capability as a character device with
root read-write access. Root processes can grant users special
privileges by allowing access to these character devices (e.g., using
chown).

UCAP character devices are located in /dev/infiniband and the class path
is /sys/class/infiniband_ucaps.

Signed-off-by: Chiara Meiohas <cmeiohas@nvidia.com>
Link: https://patch.msgid.link/5a1379187cd21178e8554afc81a3c941f21af22f.1741261611.git.leon@kernel.org


Reviewed-by: Yishai Hadas <yishaih@nvidia.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Leon Romanovsky <leon@kernel.org>