proc: subset=pid: Show /proc/self/net only for CAP_NET_ADMIN

proc: subset=pid: Show /proc/self/net only for CAP_NET_ADMIN

Cache the mounters credentials and allow access to the net directories
contingent of the permissions of the mounter of proc.

Do not show /proc/self/net when proc is mounted with subset=pid option
and the mounter does not have CAP_NET_ADMIN. To avoid inadvertently
allowing access to /proc/<pid>/net, updating mounter credentials is not
supported.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://patch.msgid.link/d2466fe9085367f1e24693c437ecb8cff2789660.1777278334.git.legion@kernel.org
Reviewed-by: Aleksa Sarai <aleksa@amutable.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>