repo: Optimize bare-user content object reads a bit

repo: Optimize bare-user content object reads a bit

`perf record ostree checkout ...` for a bare-user repo was telling
me we were spending a good 13% of our time in the depchain of `ot_lgexattrat()`.
The problem here is that traversing the `/proc` path turns out to be
somewhat expensive - there are LSM (SELinux) checks, etc.

For regular files, opening and just getting the xattr, then closing is still
quite cheap. For symlinks, we'll always need to open anyways.

This appears to shave about ~0.1 seconds off of a checkout of
`fedora-atomic/25/x86_64/docker-host` on my workstation.

Oh, and this was the last user of `ot_lgexattrat()` so we can kill it, which is
nice - the xattr code should really live in libglnx.

Closes: #796
Approved by: jlebon