sd-device: replace lstat() + open() with open(O_NOFOLLOW)

Coverity was complaining about TOCTOU (CID #745806). Indeed, it seems better
to open the file and avoid the stat altogether:

- O_NOFOLLOW means we'll get ELOOP, which we can translate to EINVAL as before,
- similarly, open(O_WRONLY) on a directory will fail with EISDIR,
- and finally, it makes no sense to check access mode ourselves: just let
  the kernel do it and propagate the error.

v2:
- fix memleak, don't clober input arg

(cherry picked from commit 2fa4861a)