readCon speedup

[kolyshkin]

When looking at runc strace output, I noticed that selinux attr files
are read byte by byte:

read(8, "u", 1) = 1
read(8, "n", 1) = 1
....

Apparently, this happens because fmt.Fscanf("%s") works that way.

Let's read the whole file via ioutil.ReadAll instead.

The added benchmark shows a drastic improvement:

name            old time/op    new time/op    delta
CurrentLabel-4    26.4µs ± 1%     4.3µs ± 1%   -83.73%  (p=0.004 n=6+5)

name            old alloc/op   new alloc/op   delta
CurrentLabel-4      352B ± 0%      776B ± 0%  +120.45%  (p=0.002 n=6+6)

name            old allocs/op  new allocs/op  delta
CurrentLabel-4      9.00 ± 0%      7.00 ± 0%   -22.22%  (p=0.002 n=6+6)

Note that increased allocation size is caused by the fact that ReadAll
preallocates a 512 bytes buffer, which is very reasonable.

[rhatdan]

LGTM

[vrothberg]

LGTM