selinux: fix labeling of /proc/net inodes

The proc net rewrite had a side effect on selinux, leading it to mislabel
the /proc/net inodes, thereby leading to incorrect denials.  Fix
security_genfs_sid to ignore extra leading / characters in the path supplied
by selinux_proc_get_sid since we now get "//net/..." rather than "/net/...".

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>