mesh: Fix keyring app keys directory iteration

mesh: Fix keyring app keys directory iteration

This fixes how app key files are accessed when finalizing
Key Refresh procedure. Instead of using open(entry->d_name, ...)
to get file descriptor, use openat(dir_fd, entry->d_name, ...)
since entry->d_name contains a relative app key filename, not an
absolute path.

Change-Id: I943a3b180ed8425d3437a659c50461e167353bfd
Signed-off-by: Anupam Roy <anupam.r@samsung.com>