UPSTREAM: security: selinux: allow per-file labeling for bpffs

Add support for genfscon per-file labeling of bpffs files. This allows
for separate permissions for different pinned bpf objects, which may
be completely unrelated to each other.

Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: Steven Moreland <smoreland@google.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>
(cherry picked from commit 4ca54d3d3022ce27170b50e4bdecc3a42f05dbdc)
[which is v5.6-rc1-10-g4ca54d3d3022 and thus already included in 5.10]
Bug: 200440527
Change-Id: I8234b9047f29981b8140bd81bb2ff070b3b0b843
(cherry picked from commit d52ac987ad2ae16ff313d7fb6185bc412cb221a4)