Avoid memory allocation while opening smackfs files.

Using openat() on pre-opened smackfs directory eliminates need to
construct absolute path to a smackfs file before opening it.

Other than it improves availability because file descriptor is kept
open to the mount point throughout the life-cycle of the process.
And it also improves security because as long as the file descriptor
is valid, files really come from SmackFS.

[jsakkine: updated commit message]

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>