brcompat_mod: Check if user has CAP_NET_ADMIN in ioctl handler

brcompat_mod: Check if user has CAP_NET_ADMIN in ioctl handler

This patch checks that the user calling ioctl() to create, delete, or
modify bridges has the CAP_NET_ADMIN capability. This prevents
unpriviledged users from modifying the bridge configuration through
brcompatd. The checks are actually the same performed in
net/bridge/br_ioctl.c by the Linux kernel.

Signed-off-by: Vivien Bernet-Rollande <vbr@soprive.net>
Signed-off-by: Jesse Gross <jesse@nicira.com>